// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: MIT

package spmd

import (
	"encoding/binary"
	"math"

	"sourcedock.dev/petrbalvin/tensor/internal/base"
	"sourcedock.dev/petrbalvin/tensor/internal/core"
)

// Op names a reduction's arithmetic.
type Op uint8

const (
	// Sum adds: shard partials through the balanced tree, rank arrays
	// in rank order.
	Sum Op = iota
	// Min takes the smallest; NaN never wins.
	Min
	// Max takes the largest; NaN never wins.
	Max
	// Any reports whether any element is true; Bool arrays only.
	Any
	// All reports whether every element is true; Bool arrays only.
	All
	// Prod multiplies: shard partials through the balanced product
	// tree, rank arrays in rank order.
	Prod
)

func (o Op) String() string {
	switch o {
	case Sum:
		return "sum"
	case Min:
		return "min"
	case Max:
		return "max"
	case Any:
		return "any"
	case All:
		return "all"
	case Prod:
		return "prod"
	}
	return "?"
}

// The payload kinds a shard's contribution can travel as. The kind is
// part of the wire, so the root combines exactly what the ranks
// folded, never a guess from the dtype.
const (
	kindNone     byte = 0  // a rank whose piece is empty
	kindSumF64   byte = 1  // one float64 fold per block
	kindSumC128  byte = 2  // one complex fold per block
	kindSumI64   byte = 3  // one exact int64 sum per block
	kindExtF64   byte = 4  // one float64 extremum and candidate flag per block
	kindExtI64   byte = 5  // one exact int64 extremum per block
	kindFlag     byte = 6  // one 0/1 flag for the whole piece
	kindProdF64  byte = 7  // one float64 product per block
	kindProdF32  byte = 8  // one native float32 product per block
	kindProdHalf byte = 9  // one exact half product per block, in float64
	kindProdI64  byte = 10 // one exact int64 product per block
	kindArg      byte = 11 // one extremum candidate: value, index and flags
)

// intPayload is the integer element type a shard folds exactly.
type intPayload interface {
	int64 | int8 | uint8 | int16 | uint16 | int32 | uint32
}

// AllReduceShards answers the reduction of one global array whose
// canonical pieces the ranks hold, every rank receiving the answer.
// Each rank folds its own blocks with the single block fold, the block
// values gather at rank 0, and core's own tree and extremum rules
// combine them: the answer is the single-array reduction's exact bits,
// whatever the world's size, whatever the order the frames arrive in.
// Any and All answer the integer-class scalar, 1 or 0.
func (w *World) AllReduceShards(local *core.Array, span Span, op Op) (core.Scalar, error) {
	ans, err := w.reduceShards(local, span, op, 0)
	if err != nil {
		return core.Scalar{}, err
	}
	return w.broadcastScalar(ans, 0)
}

// ReduceShards is AllReduceShards with the answer on the root alone;
// every other rank receives nil.
func (w *World) ReduceShards(local *core.Array, span Span, op Op, root int) (*core.Scalar, error) {
	if err := checkRoot(root, w.size); err != nil {
		return nil, w.fail(err)
	}
	ans, err := w.reduceShards(local, span, op, root)
	if err != nil {
		return nil, err
	}
	if w.rank != root {
		return nil, nil
	}
	return &ans, nil
}

// blockValues is one rank's contribution to a sharded reduction: the
// index of its first block, how many blocks it covers, the payload
// kind, and the values.
type blockValues struct {
	first int
	kind  byte
	f     []float64
	f32   []float32
	c     []complex128
	i     []int64
	oks   []bool
}

// blocks reports how many partition blocks the piece covers.
func (bv blockValues) blocks() int {
	switch bv.kind {
	case kindSumF64, kindSumI64:
		return len(bv.f) + len(bv.i)
	case kindSumC128:
		return len(bv.c)
	case kindExtF64:
		return len(bv.oks)
	case kindExtI64:
		return len(bv.i)
	case kindProdF64:
		return len(bv.f)
	case kindProdF32:
		return len(bv.f32)
	case kindProdHalf, kindProdI64:
		return len(bv.f) + len(bv.i)
	}
	return 0
}

// spanBlocks names the partition blocks the span covers: [first, last).
// A span that the partition gave out always sits on block boundaries.
func spanBlocks(span Span) (first, last, parts int) {
	parts = core.FoldParts(span.Global)
	first, last = -1, -1
	for c := 0; c <= parts; c++ {
		b := core.FoldBoundary(span.Global, c)
		if b == span.Lo && first < 0 {
			first = c
		}
		if b == span.Hi {
			last = c
		}
	}
	return first, last, parts
}

// reduceShards runs the sharded reduction; the answer exists at the
// root.
func (w *World) reduceShards(local *core.Array, span Span, op Op, root int) (core.Scalar, error) {
	if err := w.status(); err != nil {
		return core.Scalar{}, err
	}
	if err := w.checkSpan(span, local); err != nil {
		return core.Scalar{}, w.fail(err)
	}
	if span.Global == 0 {
		// An empty global axis answers without an exchange: the
		// single-array reduction answers the same constants, so the
		// contract holds at the degenerate length too. The dtype
		// validates first, exactly as a non-empty array would.
		switch op {
		case Prod:
			if dt := local.Dtype(); dt == core.Bool || dt == core.Int8 || dt == core.Uint8 || dt == core.Int16 || dt == core.Uint16 || dt == core.Int32 || dt == core.Uint32 || dt == core.Complex {
				return core.Scalar{}, w.fail(base.Errf("spmd: prod of dtype %s is not supported; convert with Astype", dt))
			}
		case Min, Max:
			if dt := local.Dtype(); dt == core.Bool || dt == core.Complex {
				return core.Scalar{}, w.fail(base.Errf("spmd: %s of dtype %s has no ordering", op, dt))
			}
		case Any, All:
			if dt := local.Dtype(); dt != core.Bool {
				return core.Scalar{}, w.fail(base.Errf("spmd: %s needs Bool elements, got %s", op, dt))
			}
		}
		switch op {
		case Sum:
			switch local.Dtype() {
			case core.Complex:
				return core.ComplexScalar(0), nil
			case core.Int, core.Int8, core.Uint8, core.Int16, core.Uint16,
				core.Int32, core.Uint32, core.Bool:
				return core.IntScalar(0), nil
			default:
				return core.FloatScalar(0), nil
			}
		case Any:
			return core.IntScalar(0), nil
		case All:
			return core.IntScalar(1), nil
		case Prod:
			// The empty product is the multiplicative identity, the
			// answer the single-array product keeps.
			if local.Dtype() == core.Int {
				return core.IntScalar(1), nil
			}
			return core.FloatScalar(1), nil
		default:
			return core.Scalar{}, w.fail(base.Errf("spmd: %s of an empty array has no answer", op))
		}
	}
	bv, err := w.foldBlocks(local, span, op)
	if err != nil {
		return core.Scalar{}, w.fail(err)
	}
	if w.rank != root {
		if err := w.sendTo(root, tagShardsValues, encodeBlockValues(bv)); err != nil {
			return core.Scalar{}, err
		}
		return core.Scalar{}, nil
	}
	all := make([]blockValues, w.size)
	all[w.rank] = bv
	for r := range w.size {
		if r == root {
			continue
		}
		data, err := w.recvFrom(r, tagShardsValues)
		if err != nil {
			return core.Scalar{}, err
		}
		if all[r], err = decodeBlockValues(data); err != nil {
			return core.Scalar{}, w.fail(err)
		}
	}
	return w.combineShards(all, span, op)
}

// foldBlocks computes the rank's own blocks' values with the same
// per-block arithmetic the single-array fold uses, dtype by dtype.
func (w *World) foldBlocks(local *core.Array, span Span, op Op) (blockValues, error) {
	first, last, _ := spanBlocks(span)
	if first < 0 || last < 0 || first > last {
		return blockValues{}, base.Errf("spmd: rank %d's span [%d, %d) is not on the partition's block boundaries",
			w.rank, span.Lo, span.Hi)
	}
	bv := blockValues{first: first}
	if first == last {
		return bv, nil
	}
	n := span.Global
	count := last - first
	blockF64 := func(c int) []float64 {
		return sliceBlock(local.RawFloats()[:local.Len()], n, first, c)
	}
	switch op {
	case Sum:
		switch local.Dtype() {
		case core.Float:
			bv.kind = kindSumF64
			bv.f = make([]float64, count)
			for c := first; c < last; c++ {
				bv.f[c-first] = core.FoldRange(blockF64(c))
			}
		case core.Float32:
			bv.kind = kindSumF64
			bv.f = make([]float64, count)
			for c := first; c < last; c++ {
				bv.f[c-first] = core.FoldRangeF32(sliceBlock(local.RawFloat32s()[:local.Len()], n, first, c))
			}
		case core.Float16:
			bv.kind = kindSumF64
			bv.f = make([]float64, count)
			for c := first; c < last; c++ {
				bv.f[c-first] = core.FoldRangeF16(sliceBlock(local.RawHalves()[:local.Len()], n, first, c))
			}
		case core.Complex:
			bv.kind = kindSumC128
			bv.c = make([]complex128, count)
			for c := first; c < last; c++ {
				bv.c[c-first] = core.FoldRangeC128(sliceBlock(local.RawComplexes()[:local.Len()], n, first, c))
			}
		case core.Int:
			bv.kind = kindSumI64
			bv.i = exactSums(local.RawInts()[:local.Len()], n, first, last)
		case core.Int8:
			bv.kind = kindSumI64
			bv.i = exactSums(local.RawInt8s()[:local.Len()], n, first, last)
		case core.Uint8:
			bv.kind = kindSumI64
			bv.i = exactSums(local.RawUint8s()[:local.Len()], n, first, last)
		case core.Int16:
			bv.kind = kindSumI64
			bv.i = exactSums(local.RawInt16s()[:local.Len()], n, first, last)
		case core.Uint16:
			bv.kind = kindSumI64
			bv.i = exactSums(local.RawUint16s()[:local.Len()], n, first, last)
		case core.Int32:
			bv.kind = kindSumI64
			bv.i = exactSums(local.RawInt32s()[:local.Len()], n, first, last)
		case core.Uint32:
			bv.kind = kindSumI64
			bv.i = exactSums(local.RawUint32s()[:local.Len()], n, first, last)
		case core.Bool:
			bv.kind = kindSumI64
			src := local.RawBools()[:local.Len()]
			bv.i = make([]int64, count)
			for c := first; c < last; c++ {
				var s int64
				for _, v := range sliceBlock(src, n, first, c) {
					if v {
						s++
					}
				}
				bv.i[c-first] = s
			}
		default:
			return blockValues{}, base.Errf("spmd: sum of dtype %s is not a reduction", local.Dtype())
		}
	case Min, Max:
		greater := op == Max
		switch local.Dtype() {
		case core.Float:
			bv.kind = kindExtF64
			bv.f = make([]float64, count)
			bv.oks = make([]bool, count)
			for c := first; c < last; c++ {
				bv.f[c-first], bv.oks[c-first] = core.ExtremeRange(blockF64(c), greater)
			}
		case core.Float32:
			bv.kind = kindExtF64
			bv.f = make([]float64, count)
			bv.oks = make([]bool, count)
			for c := first; c < last; c++ {
				bv.f[c-first], bv.oks[c-first] = core.ExtremeRangeF32(sliceBlock(local.RawFloat32s()[:local.Len()], n, first, c), greater)
			}
		case core.Float16:
			bv.kind = kindExtF64
			bv.f = make([]float64, count)
			bv.oks = make([]bool, count)
			for c := first; c < last; c++ {
				bv.f[c-first], bv.oks[c-first] = core.ExtremeRangeF16(sliceBlock(local.RawHalves()[:local.Len()], n, first, c), greater)
			}
		case core.Int:
			bv.kind = kindExtI64
			bv.i = exactExtremes(local.RawInts()[:local.Len()], n, first, last, greater)
		case core.Int8:
			bv.kind = kindExtI64
			bv.i = exactExtremes(local.RawInt8s()[:local.Len()], n, first, last, greater)
		case core.Uint8:
			bv.kind = kindExtI64
			bv.i = exactExtremes(local.RawUint8s()[:local.Len()], n, first, last, greater)
		case core.Int16:
			bv.kind = kindExtI64
			bv.i = exactExtremes(local.RawInt16s()[:local.Len()], n, first, last, greater)
		case core.Uint16:
			bv.kind = kindExtI64
			bv.i = exactExtremes(local.RawUint16s()[:local.Len()], n, first, last, greater)
		case core.Int32:
			bv.kind = kindExtI64
			bv.i = exactExtremes(local.RawInt32s()[:local.Len()], n, first, last, greater)
		case core.Uint32:
			bv.kind = kindExtI64
			bv.i = exactExtremes(local.RawUint32s()[:local.Len()], n, first, last, greater)
		default:
			return blockValues{}, base.Errf("spmd: %s of dtype %s has no ordering", op, local.Dtype())
		}
	case Prod:
		switch local.Dtype() {
		case core.Float:
			bv.kind = kindProdF64
			bv.f = make([]float64, count)
			for c := first; c < last; c++ {
				bv.f[c-first] = core.FoldProd(sliceBlock(local.RawFloats()[:local.Len()], n, first, c))
			}
		case core.Float32:
			bv.kind = kindProdF32
			bv.f32 = make([]float32, count)
			for c := first; c < last; c++ {
				bv.f32[c-first] = core.FoldProdF32(sliceBlock(local.RawFloat32s()[:local.Len()], n, first, c))
			}
		case core.Float16:
			bv.kind = kindProdHalf
			bv.f = make([]float64, count)
			for c := first; c < last; c++ {
				bv.f[c-first] = core.FoldProdF16(sliceBlock(local.RawHalves()[:local.Len()], n, first, c))
			}
		case core.Int:
			bv.kind = kindProdI64
			bv.i = make([]int64, count)
			for c := first; c < last; c++ {
				bv.i[c-first] = core.FoldProdI64(sliceBlock(local.RawInts()[:local.Len()], n, first, c))
			}
		default:
			return blockValues{}, base.Errf("spmd: prod of dtype %s is not supported; convert with Astype", local.Dtype())
		}
	case Any, All:
		if local.Dtype() != core.Bool {
			return blockValues{}, base.Errf("spmd: %s needs Bool elements, got %s", op, local.Dtype())
		}
		bv.kind = kindFlag
		flag := op == All
		for _, v := range local.RawBools()[:local.Len()] {
			if op == Any && v {
				flag = true
				break
			}
			if op == All && !v {
				flag = false
				break
			}
		}
		bv.i = []int64{bToF(flag)}
	default:
		return blockValues{}, base.Errf("spmd: unknown reduction op %s", op)
	}
	return bv, nil
}

// sliceBlock is block c of the canonical partition of n elements, seen
// inside a slab whose first block is first.
func sliceBlock[T any](src []T, n, first, c int) []T {
	baseIdx := core.FoldBoundary(n, first)
	return src[core.FoldBoundary(n, c)-baseIdx : core.FoldBoundary(n, c+1)-baseIdx]
}

// exactSums folds each of the piece's blocks into an exact int64 sum.
func exactSums[T intPayload](src []T, n, first, last int) []int64 {
	out := make([]int64, last-first)
	for c := first; c < last; c++ {
		var s int64
		for _, v := range sliceBlock(src, n, first, c) {
			s += int64(v)
		}
		out[c-first] = s
	}
	return out
}

// exactExtremes folds each of the piece's blocks into an exact int64
// extremum: the values compare exactly, so the combine needs no flags.
func exactExtremes[T intPayload](src []T, n, first, last int, greater bool) []int64 {
	out := make([]int64, last-first)
	for c := first; c < last; c++ {
		blk := sliceBlock(src, n, first, c)
		best := int64(blk[0])
		for _, v := range blk[1:] {
			if w := int64(v); (greater && w > best) || (!greater && w < best) {
				best = w
			}
		}
		out[c-first] = best
	}
	return out
}

// payloadKind names the kind every non-empty piece carries; the pieces
// agree because the program runs one op on one dtype, and the root, whose
// own piece may be empty, reads it from the first rank that folded
// anything.
func payloadKind(all []blockValues) (byte, error) {
	kind := kindNone
	for _, bv := range all {
		if bv.kind == kindNone {
			continue
		}
		if kind == kindNone {
			kind = bv.kind
		} else if bv.kind != kind {
			return 0, base.Errf("spmd: the shards disagree on the payload kind: %d against %d", bv.kind, kind)
		}
	}
	return kind, nil
}

// combineShards lays the ranks' pieces over the whole partition and
// combines them with the same functions the single-array reduction
// combines its chunks with.
func (w *World) combineShards(all []blockValues, span Span, op Op) (core.Scalar, error) {
	first, last, parts := spanBlocks(span)
	if first < 0 || last < 0 {
		return core.Scalar{}, base.Errf("spmd: the root's span is not on the partition's boundaries")
	}
	kind, kindErr := payloadKind(all)
	if kindErr != nil {
		return core.Scalar{}, kindErr
	}
	if op != Any && op != All {
		covered := 0
		for _, bv := range all {
			covered += bv.blocks()
		}
		if covered != parts {
			return core.Scalar{}, base.Errf("spmd: the shards' %d blocks do not cover the partition's %d", covered, parts)
		}
		// The root joins in rank order, which is the partition's
		// order only when every piece starts at its own rank's first
		// block; a piece naming any other start would scramble the
		// tree's addends into a wrong number returned as success.
		for r, bv := range all {
			if bv.kind == kindNone || bv.kind == kindFlag {
				continue
			}
			if bv.first != r*parts/w.size {
				return core.Scalar{}, base.Errf("spmd: rank %d's piece starts at block %d, the partition puts it at %d", r, bv.first, r*parts/w.size)
			}
		}
	}
	switch op {
	case Sum:
		switch kind {
		case kindSumF64:
			vals := make([]float64, 0, parts)
			for _, bv := range all {
				vals = append(vals, bv.f...)
			}
			return core.FloatScalar(core.TreeSum(vals)), nil
		case kindSumC128:
			vals := make([]complex128, 0, parts)
			for _, bv := range all {
				vals = append(vals, bv.c...)
			}
			return core.ComplexScalar(core.TreeSum(vals)), nil
		case kindSumI64:
			vals := make([]int64, 0, parts)
			for _, bv := range all {
				vals = append(vals, bv.i...)
			}
			return core.IntScalar(core.TreeSum(vals)), nil
		default:
			return core.Scalar{}, base.Errf("spmd: sum shards disagree on the payload kind")
		}
	case Min, Max:
		greater := op == Max
		switch kind {
		case kindExtF64:
			vals := make([]float64, 0, parts)
			oks := make([]bool, 0, parts)
			for _, bv := range all {
				vals = append(vals, bv.f...)
				oks = append(oks, bv.oks...)
			}
			return core.FloatScalar(core.CombineExtrema(vals, oks, greater)), nil
		case kindExtI64:
			var best int64
			have := false
			for _, bv := range all {
				for _, v := range bv.i {
					if !have || (greater && v > best) || (!greater && v < best) {
						best, have = v, true
					}
				}
			}
			return core.IntScalar(best), nil
		default:
			return core.Scalar{}, base.Errf("spmd: %s shards disagree on the payload kind", op)
		}
	case Prod:
		switch kind {
		case kindProdF64:
			vals := make([]float64, 0, parts)
			for _, bv := range all {
				vals = append(vals, bv.f...)
			}
			return core.FloatScalar(core.TreeProd(vals)), nil
		case kindProdF32:
			vals := make([]float32, 0, parts)
			for _, bv := range all {
				vals = append(vals, bv.f32...)
			}
			return core.FloatScalar(float64(core.TreeProd(vals))), nil
		case kindProdHalf:
			vals := make([]float64, 0, parts)
			for _, bv := range all {
				vals = append(vals, bv.f...)
			}
			return core.FloatScalar(core.TreeProdHalf(vals)), nil
		case kindProdI64:
			vals := make([]int64, 0, parts)
			for _, bv := range all {
				vals = append(vals, bv.i...)
			}
			return core.IntScalar(core.TreeProd(vals)), nil
		default:
			return core.Scalar{}, base.Errf("spmd: prod shards disagree on the payload kind")
		}
	case Any, All:
		flag := op == All
		for _, bv := range all {
			if bv.kind != kindFlag || len(bv.i) == 0 {
				continue
			}
			v := bv.i[0] != 0
			if op == Any && v {
				flag = true
			}
			if op == All && !v {
				flag = false
			}
		}
		return core.IntScalar(bToF(flag)), nil
	}
	return core.Scalar{}, base.Errf("spmd: unknown reduction op %s", op)
}

// bToF carries a boolean in the int64 slot the integer-class scalar
// answers through.
func bToF(b bool) int64 {
	if b {
		return 1
	}
	return 0
}

// The shard contribution's wire form: first block index, payload kind,
// block count, then the values. Every field is checked against every
// other on the way back in.

func encodeBlockValues(bv blockValues) []byte {
	buf := binary.LittleEndian.AppendUint64(nil, uint64(bv.first))
	buf = append(buf, bv.kind)
	buf = binary.LittleEndian.AppendUint64(buf, uint64(len(bv.f)))
	buf = binary.LittleEndian.AppendUint64(buf, uint64(len(bv.f32)))
	buf = binary.LittleEndian.AppendUint64(buf, uint64(len(bv.c)))
	buf = binary.LittleEndian.AppendUint64(buf, uint64(len(bv.i)))
	for _, v := range bv.f {
		buf = binary.LittleEndian.AppendUint64(buf, math.Float64bits(v))
	}
	for _, v := range bv.c {
		buf = binary.LittleEndian.AppendUint64(buf, math.Float64bits(real(v)))
		buf = binary.LittleEndian.AppendUint64(buf, math.Float64bits(imag(v)))
	}
	for _, v := range bv.i {
		buf = binary.LittleEndian.AppendUint64(buf, uint64(v))
	}
	for _, v := range bv.f32 {
		buf = binary.LittleEndian.AppendUint32(buf, math.Float32bits(v))
	}
	for _, ok := range bv.oks {
		b := byte(0)
		if ok {
			b = 1
		}
		buf = append(buf, b)
	}
	return buf
}

func decodeBlockValues(data []byte) (blockValues, error) {
	const head = 8 + 1 + 8*4
	if len(data) < head {
		return blockValues{}, base.Errf("spmd: a shard contribution of %d bytes is shorter than its head", len(data))
	}
	// Every count is bounded as its unsigned wire value, before any
	// signed conversion: a top-bit count would come back negative and
	// sneak past an upper-bound check.
	firstU := binary.LittleEndian.Uint64(data)
	if firstU > math.MaxInt {
		return blockValues{}, base.Errf("spmd: a shard contribution names a first block of %d", firstU)
	}
	bv := blockValues{first: int(firstU)}
	bv.kind = data[8]
	nf, n32, nc, ni, err := decodeCounts(data[9:])
	if err != nil {
		return blockValues{}, err
	}
	data = data[head:]
	want := (nf+ni)*8 + n32*4 + nc*16
	if bv.kind == kindExtF64 {
		want += nf
	}
	if bv.kind == kindArg {
		want += 2
	}
	if len(data) != want {
		return blockValues{}, base.Errf("spmd: a shard contribution carries %d value bytes for %d floats, %d float32s, %d complexes and %d integers",
			len(data), nf, n32, nc, ni)
	}
	bv.f = make([]float64, nf)
	for i := range bv.f {
		bv.f[i] = math.Float64frombits(binary.LittleEndian.Uint64(data[i*8:]))
	}
	data = data[nf*8:]
	bv.c = make([]complex128, nc)
	for i := range bv.c {
		re := math.Float64frombits(binary.LittleEndian.Uint64(data[i*16:]))
		im := math.Float64frombits(binary.LittleEndian.Uint64(data[i*16+8:]))
		bv.c[i] = complex(re, im)
	}
	data = data[nc*16:]
	bv.i = make([]int64, ni)
	for i := range bv.i {
		bv.i[i] = int64(binary.LittleEndian.Uint64(data[i*8:]))
	}
	data = data[ni*8:]
	bv.f32 = make([]float32, n32)
	for i := range bv.f32 {
		bv.f32[i] = math.Float32frombits(binary.LittleEndian.Uint32(data[i*4:]))
	}
	data = data[n32*4:]
	if bv.kind == kindExtF64 {
		bv.oks = make([]bool, nf)
		for i := range bv.oks {
			switch data[i] {
			case 0:
			case 1:
				bv.oks[i] = true
			default:
				return blockValues{}, base.Errf("spmd: a shard's candidate flag %d is neither 0 nor 1", data[i])
			}
		}
	}
	if bv.kind == kindArg {
		bv.oks = make([]bool, 2)
		for i := range bv.oks {
			switch data[i] {
			case 0:
			case 1:
				bv.oks[i] = true
			default:
				return blockValues{}, base.Errf("spmd: a shard's candidate flag %d is neither 0 nor 1", data[i])
			}
		}
	}
	return bv, nil
}

// decodeCounts reads the four block counts as unsigned wire values
// and bounds each before any signed conversion: a top-bit count would
// come back negative and sneak past an upper-bound check.
func decodeCounts(data []byte) (nf, n32, nc, ni int, err error) {
	raw := [4]uint64{
		binary.LittleEndian.Uint64(data[0:8]),
		binary.LittleEndian.Uint64(data[8:16]),
		binary.LittleEndian.Uint64(data[16:24]),
		binary.LittleEndian.Uint64(data[24:32]),
	}
	for k, u := range raw {
		if u > maxBlocks {
			return 0, 0, 0, 0, base.Errf("spmd: a shard contribution names an impossible count %d in slot %d", u, k)
		}
	}
	return int(raw[0]), int(raw[1]), int(raw[2]), int(raw[3]), nil
}

// maxBlocks bounds the counts a contribution may name before anything
// is allocated: the partition itself never exceeds this.
const maxBlocks = 1 << 12

// The scalar answer travels as one fixed frame: a kind byte, then the
// int64, float64 and complex slots, each little-endian. Only the slot
// the kind names is meaningful; every slot always travels, so the form
// has no variable length to get wrong.
const (
	scalarKindInt     byte = 1
	scalarKindFloat   byte = 2
	scalarKindComplex byte = 3
)

const scalarFrameLen = 1 + 8 + 8 + 16

func encodeScalar(s core.Scalar) []byte {
	buf := make([]byte, scalarFrameLen)
	switch {
	case s.IsComplex():
		buf[0] = scalarKindComplex
		binary.LittleEndian.PutUint64(buf[17:], math.Float64bits(real(s.Complex())))
		binary.LittleEndian.PutUint64(buf[25:], math.Float64bits(imag(s.Complex())))
	case s.IsFloat():
		buf[0] = scalarKindFloat
		binary.LittleEndian.PutUint64(buf[9:], math.Float64bits(s.Float()))
	default:
		buf[0] = scalarKindInt
		binary.LittleEndian.PutUint64(buf[1:], uint64(s.Int()))
	}
	return buf
}

func sameShape(a, b []int) bool {
	if len(a) != len(b) {
		return false
	}
	for i := range a {
		if a[i] != b[i] {
			return false
		}
	}
	return true
}

func decodeScalar(data []byte) (core.Scalar, error) {
	if len(data) != scalarFrameLen {
		return core.Scalar{}, base.Errf("spmd: a scalar answer of %d bytes is not the fixed %d", len(data), scalarFrameLen)
	}
	switch data[0] {
	case scalarKindInt:
		return core.IntScalar(int64(binary.LittleEndian.Uint64(data[1:]))), nil
	case scalarKindFloat:
		return core.FloatScalar(math.Float64frombits(binary.LittleEndian.Uint64(data[9:]))), nil
	case scalarKindComplex:
		re := math.Float64frombits(binary.LittleEndian.Uint64(data[17:]))
		im := math.Float64frombits(binary.LittleEndian.Uint64(data[25:]))
		return core.ComplexScalar(complex(re, im)), nil
	}
	return core.Scalar{}, base.Errf("spmd: a scalar answer of kind %d is none of the library's", data[0])
}

// broadcastScalar carries the root's answer to every rank.
func (w *World) broadcastScalar(s core.Scalar, root int) (core.Scalar, error) {
	if w.size == 1 {
		return s, nil
	}
	if w.rank == root {
		wire := encodeScalar(s)
		for r := range w.size {
			if r == root {
				continue
			}
			if err := w.sendTo(r, tagShardsWhole, wire); err != nil {
				return core.Scalar{}, err
			}
		}
		return s, nil
	}
	data, err := w.recvFrom(root, tagShardsWhole)
	if err != nil {
		return core.Scalar{}, err
	}
	got, err := decodeScalar(data)
	if err != nil {
		return core.Scalar{}, w.fail(err)
	}
	return got, nil
}

// Reduce folds the ranks' same-shaped arrays together and answers on
// the root alone: element e of the answer is the fold of the ranks'
// elements e in rank index order, left to right. The fold's order is
// the program's own rank order, never the arrival order, so the same
// program and data answer the same bits on one machine and across a
// cluster. Every other rank receives nil.
func (w *World) Reduce(a *core.Array, op Op, root int) (*core.Array, error) {
	if err := w.status(); err != nil {
		return nil, err
	}
	if err := checkRoot(root, w.size); err != nil {
		return nil, w.fail(err)
	}
	if err := checkArrayOp(a, op); err != nil {
		return nil, w.fail(err)
	}
	if w.size == 1 {
		return a, nil
	}
	k := w.size
	n := a.Len()
	bounds := func(c int) (int, int) { return c * n / k, (c + 1) * n / k }
	// Every rank hands its chunk c to the rank that owns c.
	for c := range k {
		if c == w.rank {
			continue
		}
		lo, hi := bounds(c)
		wire, err := encodePart(nil, a, []int{hi - lo}, lo, hi-lo)
		if err != nil {
			return nil, w.fail(err)
		}
		if err := w.sendTo(c, tagReduce, wire); err != nil {
			return nil, err
		}
	}
	// The fold runs as the chunks land: the rank's own chunk seeds the
	// accumulator, rank 0's chunk replaces it when it arrives, and
	// every later chunk joins the running accumulator at its rank's
	// turn. The chain of operands is chunk 0 left to chunk k-1 right,
	// exactly the rank order it always was, so the bits cannot move,
	// while folding one chunk overlaps the transfer of the next and
	// no rank piles the whole exchange up before it computes.
	lo, hi := bounds(w.rank)
	ownWire, err := encodePart(nil, a, []int{hi - lo}, lo, hi-lo)
	if err != nil {
		return nil, w.fail(err)
	}
	own, err := decodeWire(ownWire)
	if err != nil {
		return nil, w.fail(err)
	}
	acc := own
	for i := range k {
		chunk := own
		if i != w.rank {
			data, rerr := w.recvFrom(i, tagReduce)
			if rerr != nil {
				return nil, rerr
			}
			chunk, rerr = decodeWire(data)
			if rerr != nil {
				return nil, w.fail(rerr)
			}
			if chunk.Dtype() != own.Dtype() {
				return nil, w.fail(base.Errf("spmd: rank %d's %s disagrees with rank %d's %s",
					i, chunk.Dtype(), w.rank, own.Dtype()))
			}
		}
		if i == 0 {
			acc = chunk // the chain starts at rank 0's chunk
			continue
		}
		var ferr error
		switch op {
		case Sum:
			acc, ferr = core.Add(acc, chunk)
		case Min:
			acc, ferr = core.Minimum(acc, chunk)
		case Max:
			acc, ferr = core.Maximum(acc, chunk)
		case Any:
			acc, ferr = core.Or(acc, chunk)
		case All:
			acc, ferr = core.And(acc, chunk)
		case Prod:
			acc, ferr = core.Mul(acc, chunk)
		}
		if ferr != nil {
			return nil, w.fail(ferr)
		}
	}
	// The folded chunk travels back to the root, which joins the
	// world's chunks in rank order and reshapes to the input's shape.
	flat, err := w.gather(acc, root)
	if err != nil {
		return nil, err
	}
	if w.rank != root {
		return nil, nil
	}
	return core.Reshape(flat, a.Shape()...)
}

// AllReduce is Reduce with the answer on every rank: one world, one
// answer, identical bits everywhere.
func (w *World) AllReduce(a *core.Array, op Op) (*core.Array, error) {
	whole, err := w.Reduce(a, op, 0)
	if err != nil {
		return nil, err
	}
	return w.Broadcast(whole, 0)
}

// checkArrayOp refuses the combinations that carry no arithmetic
// before any frame moves: Bool has no sum and no ordering, complex has
// no ordering, and Any with All are Bool's alone.
func checkArrayOp(a *core.Array, op Op) error {
	dt := a.Dtype()
	switch op {
	case Sum:
		if dt == core.Bool {
			return base.Errf("spmd: sum of Bool arrays has no arithmetic; Any and All are Bool's reductions")
		}
	case Prod:
		if dt == core.Bool {
			return base.Errf("spmd: prod of Bool arrays has no arithmetic; Any and All are Bool's reductions")
		}
	case Min, Max:
		if dt == core.Bool || dt == core.Complex {
			return base.Errf("spmd: %s of dtype %s has no ordering", op, dt)
		}
	case Any, All:
		if dt != core.Bool {
			return base.Errf("spmd: %s needs Bool elements, got %s", op, dt)
		}
	default:
		return base.Errf("spmd: unknown reduction op %s", op)
	}
	return nil
}

// The sharded product, norm and dot extend the reduction families to
// the vector operations the iterative solvers live on. All three carry
// one-dimensional arrays: the canonical partition cuts the axis, the
// shards fold their own blocks with the single block kernels, and the
// combine is the same tree the single-array answers use.

// AllReduceNormShards answers the Lp norm of one global array whose
// canonical pieces the ranks hold, every rank receiving the answer.
// The power sums fold the canonical blocks with the norm fold's own
// per-element arithmetic and combine through the same balanced tree,
// so the answer is the single-array norm's exact bits at any world
// size. p must be finite and positive; the infinity norm is a maximum
// and belongs to Max, not here.
func (w *World) AllReduceNormShards(local *core.Array, span Span, p float64) (core.Scalar, error) {
	ans, err := w.reduceNormShards(local, span, p, 0)
	if err != nil {
		return core.Scalar{}, err
	}
	return w.broadcastScalar(ans, 0)
}

// ReduceNormShards is AllReduceNormShards with the answer on the root
// alone; every other rank receives nil.
func (w *World) ReduceNormShards(local *core.Array, span Span, p float64, root int) (*core.Scalar, error) {
	if err := checkRoot(root, w.size); err != nil {
		return nil, w.fail(err)
	}
	ans, err := w.reduceNormShards(local, span, p, root)
	if err != nil {
		return nil, err
	}
	if w.rank != root {
		return nil, nil
	}
	return &ans, nil
}

func (w *World) reduceNormShards(local *core.Array, span Span, p float64, root int) (core.Scalar, error) {
	if err := w.status(); err != nil {
		return core.Scalar{}, err
	}
	if math.IsNaN(p) || p <= 0 || math.IsInf(p, 1) {
		return core.Scalar{}, w.fail(base.Errf("spmd: Norm shards need a finite positive p, got %v", p))
	}
	if err := w.checkOneDimSpan(local, span); err != nil {
		return core.Scalar{}, w.fail(err)
	}
	if dt := local.Dtype(); dt != core.Float && dt != core.Float32 && dt != core.Float16 && dt != core.Int {
		return core.Scalar{}, w.fail(base.Errf("spmd: norm of dtype %s is not supported; convert with Astype", dt))
	}
	if span.Global == 0 {
		return core.FloatScalar(0), nil
	}
	sums, err := w.foldNormBlocks(local, span, p)
	if err != nil {
		return core.Scalar{}, err
	}
	if w.rank != root {
		if err := w.sendTo(root, tagShardsValues, encodeBlockValues(blockValues{first: blockFirst(span), kind: kindSumF64, f: sums})); err != nil {
			return core.Scalar{}, err
		}
		return core.Scalar{}, nil
	}
	all := make([][]float64, w.size)
	all[w.rank] = sums
	for r := range w.size {
		if r == root {
			continue
		}
		data, err := w.recvFrom(r, tagShardsValues)
		if err != nil {
			return core.Scalar{}, err
		}
		bv, err := decodeBlockValues(data)
		if err != nil {
			return core.Scalar{}, w.fail(err)
		}
		if bv.kind != kindSumF64 {
			return core.Scalar{}, w.fail(base.Errf("spmd: norm shards disagree on the payload kind"))
		}
		all[r] = bv.f
	}
	total := make([]float64, 0, core.FoldParts(span.Global))
	for _, part := range all {
		total = append(total, part...)
	}
	return core.FloatScalar(core.NormRoot(core.TreeSum(total), p)), nil
}

// foldNormBlocks computes the rank's own blocks' power sums with the
// norm fold's per-element arithmetic, dtype by dtype.
func (w *World) foldNormBlocks(local *core.Array, span Span, p float64) ([]float64, error) {
	first, last, _ := spanBlocks(span)
	if first < 0 || last < 0 || first > last {
		return nil, base.Errf("spmd: rank %d's span [%d, %d) is not on the partition's block boundaries",
			w.rank, span.Lo, span.Hi)
	}
	n := span.Global
	count := last - first
	sums := make([]float64, count)
	for c := first; c < last; c++ {
		switch local.Dtype() {
		case core.Float:
			sums[c-first] = core.FoldNormPower(sliceBlock(local.RawFloats()[:local.Len()], n, first, c), p)
		case core.Float32:
			sums[c-first] = core.FoldNormPowerF32(sliceBlock(local.RawFloat32s()[:local.Len()], n, first, c), p)
		case core.Float16:
			sums[c-first] = core.FoldNormPowerF16(sliceBlock(local.RawHalves()[:local.Len()], n, first, c), p)
		case core.Int:
			sums[c-first] = core.FoldNormPowerI64(sliceBlock(local.RawInts()[:local.Len()], n, first, c), p)
		default:
			return nil, base.Errf("spmd: norm of dtype %s is not supported; convert with Astype", local.Dtype())
		}
	}
	return sums, nil
}

// blockFirst names the partition block a span starts on.
func blockFirst(span Span) int {
	first, _, _ := spanBlocks(span)
	return first
}

// AllReduceDotShards answers the dot product of two global arrays the
// ranks hold as the same canonical pieces, every rank receiving the
// answer. The shards fold their own blocks with the single block dot
// kernel and the block values combine through the same balanced tree,
// so the answer is the single-array Dot's exact bits at any world
// size. The two arrays must carry the same dtype.
func (w *World) AllReduceDotShards(x, y *core.Array, span Span) (core.Scalar, error) {
	ans, err := w.reduceDotShards(x, y, span, 0)
	if err != nil {
		return core.Scalar{}, err
	}
	return w.broadcastScalar(ans, 0)
}

// ReduceDotShards is AllReduceDotShards with the answer on the root
// alone; every other rank receives nil.
func (w *World) ReduceDotShards(x, y *core.Array, span Span, root int) (*core.Scalar, error) {
	if err := checkRoot(root, w.size); err != nil {
		return nil, w.fail(err)
	}
	ans, err := w.reduceDotShards(x, y, span, root)
	if err != nil {
		return nil, err
	}
	if w.rank != root {
		return nil, nil
	}
	return &ans, nil
}

func (w *World) reduceDotShards(x, y *core.Array, span Span, root int) (core.Scalar, error) {
	if err := w.status(); err != nil {
		return core.Scalar{}, err
	}
	if err := w.checkOneDimSpan(x, span); err != nil {
		return core.Scalar{}, w.fail(err)
	}
	if y.NDim() != 1 || y.Len() != span.Len() {
		return core.Scalar{}, w.fail(base.Errf("spmd: the second shard leads with %d elements against the span's %d", y.Len(), span.Len()))
	}
	if x.Dtype() != y.Dtype() {
		return core.Scalar{}, w.fail(base.Errf("spmd: Dot shards carry %s against %s; convert with Astype", x.Dtype(), y.Dtype()))
	}
	switch x.Dtype() {
	case core.Int:
	case core.Float, core.Float32, core.Float16, core.Complex:
	default:
		return core.Scalar{}, w.fail(base.Errf("spmd: dot of dtype %s is not supported; convert with Astype", x.Dtype()))
	}
	if span.Global == 0 {
		if x.Dtype() == core.Int {
			return core.IntScalar(0), nil
		}
		if x.Dtype() == core.Complex {
			return core.ComplexScalar(0), nil
		}
		return core.FloatScalar(0), nil
	}
	bv, err := w.foldDotBlocks(x, y, span)
	if err != nil {
		return core.Scalar{}, w.fail(err)
	}
	if w.rank != root {
		if err := w.sendTo(root, tagShardsValues, encodeBlockValues(bv)); err != nil {
			return core.Scalar{}, err
		}
		return core.Scalar{}, nil
	}
	all := make([]blockValues, w.size)
	all[w.rank] = bv
	for r := range w.size {
		if r == root {
			continue
		}
		data, err := w.recvFrom(r, tagShardsValues)
		if err != nil {
			return core.Scalar{}, err
		}
		if all[r], err = decodeBlockValues(data); err != nil {
			return core.Scalar{}, w.fail(err)
		}
	}
	return w.combineDot(all, span)
}

// foldDotBlocks computes the rank's own blocks' dot values with the
// single block dot kernel, dtype by dtype.
func (w *World) foldDotBlocks(x, y *core.Array, span Span) (blockValues, error) {
	first, last, _ := spanBlocks(span)
	if first < 0 || last < 0 || first > last {
		return blockValues{}, base.Errf("spmd: rank %d's span [%d, %d) is not on the partition's block boundaries",
			w.rank, span.Lo, span.Hi)
	}
	n := span.Global
	count := last - first
	bv := blockValues{first: first}
	if count == 0 {
		return bv, nil
	}
	switch x.Dtype() {
	case core.Float:
		bv.kind = kindSumF64
		bv.f = make([]float64, count)
		for c := first; c < last; c++ {
			bv.f[c-first] = core.FoldDot(
				sliceBlock(x.RawFloats()[:x.Len()], n, first, c),
				sliceBlock(y.RawFloats()[:y.Len()], n, first, c))
		}
	case core.Float32:
		bv.kind = kindSumF64
		bv.f = make([]float64, count)
		for c := first; c < last; c++ {
			bv.f[c-first] = core.FoldDotF32(
				sliceBlock(x.RawFloat32s()[:x.Len()], n, first, c),
				sliceBlock(y.RawFloat32s()[:y.Len()], n, first, c))
		}
	case core.Float16:
		bv.kind = kindSumF64
		bv.f = make([]float64, count)
		for c := first; c < last; c++ {
			bv.f[c-first] = core.FoldDotF16(
				sliceBlock(x.RawHalves()[:x.Len()], n, first, c),
				sliceBlock(y.RawHalves()[:y.Len()], n, first, c))
		}
	case core.Complex:
		bv.kind = kindSumC128
		bv.c = make([]complex128, count)
		for c := first; c < last; c++ {
			bv.c[c-first] = core.FoldDotC128(
				sliceBlock(x.RawComplexes()[:x.Len()], n, first, c),
				sliceBlock(y.RawComplexes()[:y.Len()], n, first, c))
		}
	case core.Int:
		bv.kind = kindSumI64
		bv.i = make([]int64, count)
		for c := first; c < last; c++ {
			bv.i[c-first] = core.FoldDotI64(
				sliceBlock(x.RawInts()[:x.Len()], n, first, c),
				sliceBlock(y.RawInts()[:y.Len()], n, first, c))
		}
	default:
		return blockValues{}, base.Errf("spmd: dot of dtype %s is not supported; convert with Astype", x.Dtype())
	}
	return bv, nil
}

// combineDot folds the shards' dot values through the balanced tree.
func (w *World) combineDot(all []blockValues, span Span) (core.Scalar, error) {
	first, _, parts := spanBlocks(span)
	if first < 0 {
		return core.Scalar{}, base.Errf("spmd: the root's span is not on the partition's boundaries")
	}
	covered := 0
	for r, bv := range all {
		covered += bv.blocks()
		if bv.kind == kindNone {
			continue
		}
		if bv.first != r*parts/w.size {
			return core.Scalar{}, base.Errf("spmd: rank %d's piece starts at block %d, the partition puts it at %d", r, bv.first, r*parts/w.size)
		}
	}
	if covered != parts {
		return core.Scalar{}, base.Errf("spmd: the shards' %d blocks do not cover the partition's %d", covered, parts)
	}
	kind, kindErr := payloadKind(all)
	if kindErr != nil {
		return core.Scalar{}, kindErr
	}
	switch kind {
	case kindSumI64:
		vals := make([]int64, 0, parts)
		for _, bv := range all {
			vals = append(vals, bv.i...)
		}
		return core.IntScalar(core.TreeSum(vals)), nil
	case kindSumC128:
		vals := make([]complex128, 0, parts)
		for _, bv := range all {
			vals = append(vals, bv.c...)
		}
		return core.ComplexScalar(core.TreeSum(vals)), nil
	case kindSumF64:
		vals := make([]float64, 0, parts)
		for _, bv := range all {
			vals = append(vals, bv.f...)
		}
		return core.FloatScalar(core.TreeSum(vals)), nil
	default:
		return core.Scalar{}, base.Errf("spmd: dot shards disagree on the payload kind")
	}
}