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

package signal

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

import "sourcedock.dev/petrbalvin/tensor/internal/engine"

// 2-D convolution. The forward pass walks every output
// position and sums the contribution of every (cIn, kH, kW) input
// patch times the kernel weights. NCHW layout: input is (N, C_in,
// H, W), kernel is (C_out, C_in, kH, kW), output is (N, C_out, H_out,
// W_out). padding applies to both spatial dims (a single int for now);
// stride is also a single int. bias is added per output channel.
//
// Performance note: this is a direct O(N * C_out * C_in * H_out *
// W_out * kH * kW) implementation, not an im2col + GEMM. The kernels
// hold two constraints fixed:
//
//   - Tap order. Each output element accumulates its taps in exactly
//     the (cIn, kH, kW) nesting the kernels have always used; float64
//     addition is not associative, so any other order changes bits.
//     Loop interchange that keeps this per-element order (moving the
//     output position innermost) is safe; reordering taps is not.
//   - Parallel split. Work is distributed over output rows, which own
//     disjoint output elements, so no split can influence a sum.
//
// Non-float inputs widen once into a scratch payload before the loops:
// FloatAt widens exactly, so the multiplied values are bit-identical
// to the per-element accessor reads.

// Conv2D performs a 2-D convolution forward pass.
func Conv2D(input, kernel, bias *core.Array, stride, padding int) (*core.Array, error) {
	return conv2DImpl(input, kernel, bias, stride, padding, 1)
}

// Conv2DGroups performs a 2-D convolution with `groups` for grouped
// (groups=2) and depthwise (groups=C_in) convolutions. When groups=1
// the call is identical to Conv2D.
func Conv2DGroups(input, kernel, bias *core.Array, stride, padding, groups int) (*core.Array, error) {
	return conv2DImpl(input, kernel, bias, stride, padding, groups)
}

// conv2DImpl is the shared implementation behind Conv2D and
// Conv2DGroups. groups=1 is the standard dense convolution; groups>1
// is a depthwise convolution when groups == c_in, or a grouped
// convolution otherwise (the kernel's in-channels is c_in/groups per
// group).
func conv2DImpl(input, kernel, bias *core.Array, stride, padding, groups int) (*core.Array, error) {
	if input.Dtype() == core.Complex {
		return nil, base.Errf("Conv2D: complex arrays are not supported")
	}
	if kernel.Dtype() == core.Complex {
		return nil, base.Errf("Conv2D: complex kernel is not supported")
	}
	if input.NDim() != 4 {
		return nil, base.Errf("Conv2D: input must be 4-D (N, C_in, H, W), got shape %s", base.ShapeText(input.Shape()))
	}
	if kernel.NDim() != 4 {
		return nil, base.Errf("Conv2D: kernel must be 4-D (C_out, C_in, kH, kW), got shape %s", base.ShapeText(kernel.Shape()))
	}
	if stride < 1 {
		return nil, base.Errf("Conv2D: stride must be at least 1, got %d", stride)
	}
	if padding < 0 {
		return nil, base.Errf("Conv2D: padding must be non-negative, got %d", padding)
	}
	n, cIn, hIn, wIn := input.Shape()[0], input.Shape()[1], input.Shape()[2], input.Shape()[3]
	cOut, cInK, kH, kW := kernel.Shape()[0], kernel.Shape()[1], kernel.Shape()[2], kernel.Shape()[3]
	if groups < 1 {
		return nil, base.Errf("Conv2D: groups must be at least 1, got %d", groups)
	}
	if cIn%groups != 0 {
		return nil, base.Errf("Conv2D: input channels %d must be divisible by groups %d", cIn, groups)
	}
	if cOut%groups != 0 {
		return nil, base.Errf("Conv2D: output channels %d must be divisible by groups %d", cOut, groups)
	}
	cInPerGroup := cIn / groups
	cOutPerGroup := cOut / groups
	if cInK != cInPerGroup {
		return nil, base.Errf("Conv2D: kernel in-channels %d does not match cIn/groups %d", cInK, cInPerGroup)
	}
	// Guard the numerators before the division: a negative numerator
	// truncates toward zero and masquerades as a 1-wide output.
	hNum := hIn + 2*padding - kH
	wNum := wIn + 2*padding - kW
	if hNum < 0 || wNum < 0 {
		return nil, base.Errf("Conv2D: kernel %dx%d with padding %d does not fit the %dx%d input", kH, kW, padding, hIn, wIn)
	}
	hOut := hNum/stride + 1
	wOut := wNum/stride + 1
	if hOut < 1 || wOut < 1 {
		return nil, base.Errf("Conv2D: output is empty (H_out=%d, W_out=%d)", hOut, wOut)
	}
	if bias != nil && (bias.NDim() != 1 || bias.Len() != cOut) {
		return nil, base.Errf("Conv2D: bias must be 1-D of length %d, got shape %s", cOut, base.ShapeText(bias.Shape()))
	}
	// The bias widens through FloatAt like the payloads, which reads
	// only real dtypes: a complex bias is refused instead of widened.
	if bias != nil && bias.Dtype() == core.Complex {
		return nil, base.Errf("Conv2D: complex bias is not supported")
	}
	out, oerr := core.Zeros(core.Float, []int{n, cOut, hOut, wOut}...)
	if oerr != nil {
		return nil, oerr
	}
	// The fast path streams raw float64 payloads; other dtypes widen
	// once into scratch (exact, see widenFloats) so the loop below
	// never branches on dtype. Dispatching once here keeps the
	// per-element work free of accessor calls and bounds on the shape
	// of the read.
	inF := input.RawFloats()
	if inF == nil || input.Strided() {
		inF = widenFloats(input)
	}
	kerF := kernel.RawFloats()
	if kerF == nil || kernel.Strided() {
		kerF = widenFloats(kernel)
	}
	var biasF []float64
	if bias != nil {
		biasF = widenFloats(bias)
	}
	outF := out.RawFloats()
	// Blocked over output channels: one work item is one (batch, group,
	// output row, channel block) and its accumulator covers every output
	// channel of the block at once. A tap's input row then feeds every
	// channel of the block from the lowest cache instead of being read
	// again for each of them, which is what the per-channel walk spent
	// its bandwidth on. The tap walk stays (cIn, kH, kW) ascending and
	// every output element is updated by exactly one tap of the walk, so
	// each sum sees the same addends in the same order as before, and the
	// bias is added last, as before. When the row is too wide for the
	// block to stay in cache the block shrinks to one channel, which is
	// the per-channel walk again.
	ocBlock := max(1, min(cOutPerGroup, convAccCap/wOut))
	blocksPerRow := (cOutPerGroup + ocBlock - 1) / ocBlock
	// Per-channel kernel offsets within the block: the weight base of the
	// block's first channel is added per item, the rest follow from the
	// channel index, so a tap reads its weight without multiplying the
	// channel out.
	kOff := make([]int, ocBlock)
	for i := range kOff {
		kOff[i] = i * cInPerGroup * kH * kW
	}
	items := n * hOut * groups * blocksPerRow
	floor := workFloorFor(wOut * ocBlock * cInPerGroup * kH * kW)
	engine.ParallelMin(items, floor, func(bs, be int) {
		acc := engine.GetFloat64Buf(ocBlock * wOut)
		defer engine.PutFloat64Buf(acc)
		for item := bs; item < be; item++ {
			ob := item % blocksPerRow
			rest := item / blocksPerRow
			g := rest % groups
			rest /= groups
			oh := rest % hOut
			batch := rest / hOut
			oc0 := g*cOutPerGroup + ob*ocBlock
			oc1 := min(oc0+ocBlock, (g+1)*cOutPerGroup)
			clear(acc)
			chanBase := batch*cIn*hIn*wIn + g*cInPerGroup*hIn*wIn
			ocBase := oc0 * cInPerGroup * kH * kW
			for icInGroup := range cInPerGroup {
				inChan := chanBase + icInGroup*hIn*wIn
				tapBase := icInGroup * kH * kW
				for kh := range kH {
					ih := oh*stride + kh - padding
					if ih < 0 || ih >= hIn {
						continue
					}
					inRow := inChan + ih*wIn
					for kw := range kW {
						// The outputs this tap reaches: ow*stride
						// must land the input column inside the row.
						// The reach is a property of the tap alone, so
						// it is computed once for the whole block.
						lo := 0
						if d := padding - kw; d > 0 {
							lo = (d + stride - 1) / stride
						}
						hi := wOut
						if m := wIn - 1 + padding - kw; m < 0 {
							continue
						} else if t := m/stride + 1; t < hi {
							hi = t
						}
						tap := kw - padding
						if hi <= lo {
							// The tap reaches no output here (its
							// whole window lies in the padding): the
							// unit-stride sub-slices below would get
							// a high bound under their low one.
							continue
						}
						tapOff := tapBase + kh*kW + kw
						if stride == 1 {
							// Unit stride lets both sides range
							// over paired sub-slices: no bounds
							// checks and no index multiply in
							// the hot loop.
							src := inF[inRow+lo+tap : inRow+hi+tap]
							for o := range oc1 - oc0 {
								k := kerF[ocBase+kOff[o]+tapOff]
								dst := acc[o*wOut+lo : o*wOut+hi]
								for j, v := range src {
									dst[j] += v * k
								}
							}
							continue
						}
						for o := range oc1 - oc0 {
							k := kerF[ocBase+kOff[o]+tapOff]
							base := o * wOut
							for ow := lo; ow < hi; ow++ {
								acc[base+ow] += inF[inRow+ow*stride+tap] * k
							}
						}
					}
				}
			}
			for o := range oc1 - oc0 {
				oc := oc0 + o
				b := 0.0
				if biasF != nil {
					b = biasF[oc]
				}
				off := (batch*cOut + oc) * hOut * wOut
				base := o * wOut
				for ow := range wOut {
					outF[off+oh*wOut+ow] = acc[base+ow] + b
				}
			}
		}
	})
	return out, nil
}

// conv1DRowBlock is the output-length grain the 1-D kernel splits rows
// into when a row alone would starve the worker pool (a single long
// signal with a handful of output channels is exactly that case).
const conv1DRowBlock = 512

// convAccCap bounds one work item's channel-block accumulator, in
// float64 entries: a block of channels·row values is what the taps of
// a whole row feed, and it stays cheap only while it fits the lowest
// cache. Above the cap the block covers fewer channels, down to one,
// where the kernel degenerates to the per-channel walk it replaced.
// Every channel-blocked kernel shares it.
const convAccCap = 1 << 12

// Conv1D performs a 1-D convolution forward pass. input is (N, C_in,
// L), kernel is (C_out, C_in, kL), output is (N, C_out, L_out). NCL
// layout.
func Conv1D(input, kernel, bias *core.Array, stride, padding, dilation int) (*core.Array, error) {
	if input.Dtype() == core.Complex {
		return nil, base.Errf("Conv1D: complex arrays are not supported")
	}
	if kernel.Dtype() == core.Complex {
		return nil, base.Errf("Conv1D: complex kernel is not supported")
	}
	if input.NDim() != 3 {
		return nil, base.Errf("Conv1D: input must be 3-D (N, C_in, L), got shape %s", base.ShapeText(input.Shape()))
	}
	if kernel.NDim() != 3 {
		return nil, base.Errf("Conv1D: kernel must be 3-D (C_out, C_in, kL), got shape %s", base.ShapeText(kernel.Shape()))
	}
	if stride < 1 || dilation < 1 || padding < 0 {
		return nil, base.Errf("Conv1D: stride/dilation must be ≥1, padding ≥0")
	}
	if dilation != 1 {
		return nil, base.Errf("Conv1D: dilation != 1 is not implemented yet")
	}
	n, cIn, lIn := input.Shape()[0], input.Shape()[1], input.Shape()[2]
	cOut, cInK, kL := kernel.Shape()[0], kernel.Shape()[1], kernel.Shape()[2]
	if cInK != cIn {
		return nil, base.Errf("Conv1D: kernel in-channels %d does not match input %d", cInK, cIn)
	}
	lNum := lIn + 2*padding - kL
	if lNum < 0 {
		return nil, base.Errf("Conv1D: kernel length %d with padding %d does not fit the length-%d input", kL, padding, lIn)
	}
	lOut := lNum/stride + 1
	if lOut < 1 {
		return nil, base.Errf("Conv1D: output is empty (L_out=%d)", lOut)
	}
	if bias != nil && (bias.NDim() != 1 || bias.Len() != cOut) {
		return nil, base.Errf("Conv1D: bias must be 1-D of length %d, got shape %s", cOut, base.ShapeText(bias.Shape()))
	}
	if bias != nil && bias.Dtype() == core.Complex {
		return nil, base.Errf("Conv1D: complex bias is not supported")
	}
	out, oerr := core.Zeros(core.Float, []int{n, cOut, lOut}...)
	if oerr != nil {
		return nil, oerr
	}
	inF := input.RawFloats()
	if inF == nil || input.Strided() {
		inF = widenFloats(input)
	}
	kerF := kernel.RawFloats()
	if kerF == nil || kernel.Strided() {
		kerF = widenFloats(kernel)
	}
	var biasF []float64
	if bias != nil {
		biasF = widenFloats(bias)
	}
	outF := out.RawFloats()
	// Work items are output row segments (batch, oc, a block of ol).
	// Segmenting keeps a single long signal with few output channels
	// from starving the pool; segments are disjoint, and each output
	// accumulates its taps in the (cIn, kL) order. The per-tap spans
	// are shape-only and are clipped to the row block per item.
	spans := convRowSpans(kL, padding, stride, lIn, lOut)
	blocks := (lOut + conv1DRowBlock - 1) / conv1DRowBlock
	items := n * cOut * blocks
	engine.ParallelMin(items, workFloorFor(min(conv1DRowBlock, lOut)*cIn*kL), func(bs, be int) {
		acc := engine.GetFloat64Buf(min(conv1DRowBlock, lOut))
		defer engine.PutFloat64Buf(acc)
		for item := bs; item < be; item++ {
			block := item % blocks
			row := item / blocks
			oc := row % cOut
			batch := row / cOut
			ols := block * conv1DRowBlock
			ole := min(ols+conv1DRowBlock, lOut)
			clear(acc)
			chanBase := batch * cIn * lIn
			kerBase := oc * cIn * kL
			// The interior of the block reached by every tap of the
			// kernel: [fa, fb) gets one fused sweep per input channel
			// with the running sum held in a register, instead of one
			// accumulator pass per tap. Each element still receives
			// its taps in (cIn, kL) order, so the accumulated bits
			// are unchanged. Outside the register-sized kernel rows
			// the per-tap walk below covers the whole block.
			fa := max(spans[0].lo, ols)
			fb := min(spans[kL-1].hi, ole)
			fused := stride == 1 && kL >= 2 && kL <= 7 && fb > fa
			for ic := range cIn {
				inRow := chanBase + ic*lIn
				kerRow := kerBase + ic*kL
				for kl := range kL {
					sp := spans[kl]
					if !sp.valid {
						continue
					}
					k := kerF[kerRow+kl]
					// Clip the tap's precomputed span to this row
					// block and to the edges of the fused region;
					// the clip only narrows.
					lo := max(sp.lo, ols)
					hi := min(sp.hi, ole)
					if fused {
						hi = min(hi, fa)
					}
					tap := sp.tap
					if hi > lo {
						if stride == 1 {
							// Unit stride: paired sub-slices
							// keep the hot loop free of bounds
							// checks and index multiplies.
							src := inF[inRow+lo+tap : inRow+hi+tap]
							dst := acc[lo-ols : hi-ols]
							for j, v := range src {
								dst[j] += v * k
							}
						} else {
							for ol := lo; ol < hi; ol++ {
								acc[ol-ols] += inF[inRow+ol*stride+tap] * k
							}
						}
					}
					if !fused {
						continue
					}
					lo = max(sp.lo, fb)
					hi = min(sp.hi, ole)
					if hi <= lo {
						continue
					}
					src := inF[inRow+lo+tap : inRow+hi+tap]
					dst := acc[lo-ols : hi-ols]
					for j, v := range src {
						dst[j] += v * k
					}
				}
				if !fused {
					continue
				}
				// The fused interior sweep: one shifted window of the
				// input row, its sub-slices in lockstep, taps in
				// registers.
				src := inF[inRow+fa-padding : inRow+fb-padding+kL-1]
				dst := acc[fa-ols : fb-ols]
				m := len(dst)
				switch kL {
				case 2:
					k0, k1 := kerF[kerRow], kerF[kerRow+1]
					s1 := src[1 : 1+m]
					for i, v0 := range src[:m] {
						d := dst[i] + v0*k0
						dst[i] = d + s1[i]*k1
					}
				case 3:
					k0, k1, k2 := kerF[kerRow], kerF[kerRow+1], kerF[kerRow+2]
					s1 := src[1 : 1+m]
					s2 := src[2 : 2+m]
					for i, v0 := range src[:m] {
						d := dst[i] + v0*k0
						d += s1[i] * k1
						dst[i] = d + s2[i]*k2
					}
				case 4:
					k0, k1, k2, k3 := kerF[kerRow], kerF[kerRow+1], kerF[kerRow+2], kerF[kerRow+3]
					s1 := src[1 : 1+m]
					s2 := src[2 : 2+m]
					s3 := src[3 : 3+m]
					for i, v0 := range src[:m] {
						d := dst[i] + v0*k0
						d += s1[i] * k1
						d += s2[i] * k2
						dst[i] = d + s3[i]*k3
					}
				case 5:
					k0, k1, k2, k3, k4 := kerF[kerRow], kerF[kerRow+1], kerF[kerRow+2], kerF[kerRow+3], kerF[kerRow+4]
					s1 := src[1 : 1+m]
					s2 := src[2 : 2+m]
					s3 := src[3 : 3+m]
					s4 := src[4 : 4+m]
					for i, v0 := range src[:m] {
						d := dst[i] + v0*k0
						d += s1[i] * k1
						d += s2[i] * k2
						d += s3[i] * k3
						dst[i] = d + s4[i]*k4
					}
				case 6:
					k0, k1, k2, k3, k4, k5 := kerF[kerRow], kerF[kerRow+1], kerF[kerRow+2], kerF[kerRow+3], kerF[kerRow+4], kerF[kerRow+5]
					s1 := src[1 : 1+m]
					s2 := src[2 : 2+m]
					s3 := src[3 : 3+m]
					s4 := src[4 : 4+m]
					s5 := src[5 : 5+m]
					for i, v0 := range src[:m] {
						d := dst[i] + v0*k0
						d += s1[i] * k1
						d += s2[i] * k2
						d += s3[i] * k3
						d += s4[i] * k4
						dst[i] = d + s5[i]*k5
					}
				case 7:
					k0, k1, k2, k3, k4, k5, k6 := kerF[kerRow], kerF[kerRow+1], kerF[kerRow+2], kerF[kerRow+3], kerF[kerRow+4], kerF[kerRow+5], kerF[kerRow+6]
					s1 := src[1 : 1+m]
					s2 := src[2 : 2+m]
					s3 := src[3 : 3+m]
					s4 := src[4 : 4+m]
					s5 := src[5 : 5+m]
					s6 := src[6 : 6+m]
					for i, v0 := range src[:m] {
						d := dst[i] + v0*k0
						d += s1[i] * k1
						d += s2[i] * k2
						d += s3[i] * k3
						d += s4[i] * k4
						d += s5[i] * k5
						dst[i] = d + s6[i]*k6
					}
				}
			}
			b := 0.0
			if biasF != nil {
				b = biasF[oc]
			}
			off := batch*cOut*lOut + oc*lOut
			for ol := ols; ol < ole; ol++ {
				outF[off+ol] = acc[ol-ols] + b
			}
		}
	})
	return out, nil
}

// convGatherSpan is the reach of one transposed-convolution kernel
// column: the outputs it feeds sit stride apart on the phase lattice,
// at lattice indices [j0, j0+count), and read the input row from
// base on. first is the lowest output column reached and phase is its
// residue modulo the stride, which decides which accumulator partition
// the tap lands in. A tap with an empty reach is marked invalid.
type convGatherSpan struct {
	first, j0, count, base, phase int
	valid                         bool
}

// convGatherSpans precomputes the per-tap gather spans for one
// transposed kernel row. The values depend on the kernel column and
// the shape alone, so the hot loop reads them from this table instead
// of dividing per output position.
func convGatherSpans(kW, padding, stride, wIn, wOut int) []convGatherSpan {
	spans := make([]convGatherSpan, kW)
	for kw := range kW {
		lo := 0
		if d := kw - padding; d > 0 {
			lo = d
		}
		hi := wOut
		if m := wIn*stride + kw - padding; m < hi {
			hi = m
		}
		if hi <= lo {
			continue
		}
		r := (kw - padding - lo) % stride
		if r < 0 {
			r += stride
		}
		first := lo + r
		count := (hi - first + stride - 1) / stride
		if count <= 0 {
			continue
		}
		spans[kw].first = first
		spans[kw].count = count
		spans[kw].j0 = first / stride
		spans[kw].base = (first + padding - kw) / stride
		spans[kw].phase = first % stride
		spans[kw].valid = true
	}
	return spans
}

// convTapSpan is the reach of one kernel tap along an output row: the
// tap lands on outputs [lo, hi) and reads the input row offset by tap
// from them. A tap whose whole window falls into the padding is marked
// invalid, so the hot loop skips it without recomputing why.
type convTapSpan struct {
	lo, hi, tap int
	valid       bool
}

// convRowSpans precomputes the per-tap spans of one kernel row for
// stride and padding. The bounds depend on the kernel column and the
// shape alone, never on the output position, so the innermost loops
// load them from this table instead of dividing once per tap.
func convRowSpans(kW, padW, stride, wIn, wOut int) []convTapSpan {
	spans := make([]convTapSpan, kW)
	for kw := range kW {
		lo := 0
		if d := padW - kw; d > 0 {
			lo = (d + stride - 1) / stride
		}
		// lo and tap are stored whatever the reach: the fused region
		// bounds read spans[0].lo and spans[kW-1].hi, so an extreme tap
		// that reaches nothing must leave an empty window (hi 0) rather
		// than a zero lo beside a stale hi.
		spans[kw].lo = lo
		spans[kw].tap = kw - padW
		if m := wIn - 1 + padW - kw; m < 0 {
			continue
		} else if t := m/stride + 1; t < wOut {
			spans[kw].hi = t
		} else {
			spans[kw].hi = wOut
		}
		if spans[kw].hi <= lo {
			// The tap reaches no output here (its whole window
			// lies in the padding): the unit-stride sub-slices
			// would get a high bound under their low one.
			spans[kw].hi = 0
			continue
		}
		spans[kw].valid = true
	}
	return spans
}

// Conv3D performs a 3-D convolution forward pass. input is (N, C_in,
// D, H, W), kernel is (C_out, C_in, kD, kH, kW), output is (N, C_out,
// D_out, H_out, W_out). NCDHW layout.
func Conv3D(input, kernel, bias *core.Array, stride int, padding, dilation [3]int) (*core.Array, error) {
	if input.Dtype() == core.Complex {
		return nil, base.Errf("Conv3D: complex arrays are not supported")
	}
	if kernel.Dtype() == core.Complex {
		return nil, base.Errf("Conv3D: complex kernel is not supported")
	}
	if input.NDim() != 5 {
		return nil, base.Errf("Conv3D: input must be 5-D, got shape %s", base.ShapeText(input.Shape()))
	}
	if kernel.NDim() != 5 {
		return nil, base.Errf("Conv3D: kernel must be 5-D (C_out, C_in, kD, kH, kW), got shape %s", base.ShapeText(kernel.Shape()))
	}
	if stride < 1 {
		return nil, base.Errf("Conv3D: stride must be at least 1, got %d", stride)
	}
	for d, dil := range dilation {
		if dil != 1 {
			return nil, base.Errf("Conv3D: dilation != 1 not implemented yet (dim %d)", d)
		}
	}
	n, cIn, dIn, hIn, wIn := input.Shape()[0], input.Shape()[1], input.Shape()[2], input.Shape()[3], input.Shape()[4]
	cOut, cInK, kD, kH, kW := kernel.Shape()[0], kernel.Shape()[1], kernel.Shape()[2], kernel.Shape()[3], kernel.Shape()[4]
	if cInK != cIn {
		return nil, base.Errf("Conv3D: kernel in-channels %d does not match input %d", cInK, cIn)
	}
	padD, padH, padW := padding[0], padding[1], padding[2]
	// Guard the numerators before the division: a negative numerator
	// truncates toward zero and masquerades as a 1-wide output.
	dNum := dIn + 2*padD - kD
	hNum := hIn + 2*padH - kH
	wNum := wIn + 2*padW - kW
	if dNum < 0 || hNum < 0 || wNum < 0 {
		return nil, base.Errf("Conv3D: kernel %dx%dx%d with padding %v does not fit the input", kD, kH, kW, padding)
	}
	dOut := dNum/stride + 1
	hOut := hNum/stride + 1
	wOut := wNum/stride + 1
	if dOut < 1 || hOut < 1 || wOut < 1 {
		return nil, base.Errf("Conv3D: output is empty")
	}
	if bias != nil && (bias.NDim() != 1 || bias.Len() != cOut) {
		return nil, base.Errf("Conv3D: bias must be 1-D of length %d, got shape %s", cOut, base.ShapeText(bias.Shape()))
	}
	if bias != nil && bias.Dtype() == core.Complex {
		return nil, base.Errf("Conv3D: complex bias is not supported")
	}
	out, oerr := core.Zeros(core.Float, []int{n, cOut, dOut, hOut, wOut}...)
	if oerr != nil {
		return nil, oerr
	}
	strideHW := wIn
	strideDHW := hIn * wIn
	strideCDHW := dIn * hIn * wIn
	inF := input.RawFloats()
	if inF == nil || input.Strided() {
		inF = widenFloats(input)
	}
	kerF := kernel.RawFloats()
	if kerF == nil || kernel.Strided() {
		kerF = widenFloats(kernel)
	}
	var biasF []float64
	if bias != nil {
		biasF = widenFloats(bias)
	}
	outF := out.RawFloats()
	// One work item per output depth-height row (batch, oc, od, oh).
	// Rows are disjoint; the taps of one output accumulate in the
	// (cIn, kD, kH, kW) order. The channel-blocked shape was tried
	// here and measured: it paid 12 to 14 percent at 32 and 128
	// channels and lost 12 percent at 4, where the per-tap channel
	// loop costs more than the re-reads it saves, so the per-channel
	// walk stays. The per-tap column spans are shape-only, so they
	// are read from a table built once per call.
	spans := convRowSpans(kW, padW, stride, wIn, wOut)
	items := n * cOut * dOut * hOut
	engine.ParallelMin(items, workFloorFor(wOut*cIn*kD*kH*kW), func(bs, be int) {
		acc := engine.GetFloat64Buf(wOut)
		defer engine.PutFloat64Buf(acc)
		for item := bs; item < be; item++ {
			oh := item % hOut
			t := item / hOut
			od := t % dOut
			t /= dOut
			oc := t % cOut
			batch := t / cOut
			clear(acc)
			for ic := range cIn {
				inChan := batch*cIn*strideCDHW + ic*strideCDHW
				kerChan := oc*cInK*kD*kH*kW + ic*kD*kH*kW
				for kd := range kD {
					id := od*stride + kd - padD
					if id < 0 || id >= dIn {
						continue
					}
					inDepth := inChan + id*strideDHW
					kerDepth := kerChan + kd*kH*kW
					for kh := range kH {
						ih := oh*stride + kh - padH
						if ih < 0 || ih >= hIn {
							continue
						}
						inRow := inDepth + ih*strideHW
						kerRow := kerDepth + kh*kW
						if stride == 1 && kW >= 2 && kW <= 7 {
							// Unit stride with a
							// register-sized kernel row. The
							// interior [a, b) is reached by
							// every tap, so one sweep adds all
							// taps there with the running sum
							// held in a register instead of
							// re-reading the accumulator row
							// once per tap. Each element still
							// receives its taps in kw order, so
							// the accumulated bits are unchanged.
							a := spans[0].lo
							b := spans[kW-1].hi
							if b <= a {
								// No interior: an empty window
								// hands every tap's span to the
								// second part below in one
								// piece, exactly once.
								a, b = 0, 0
							}
							for kw := range kW {
								sp := spans[kw]
								if !sp.valid {
									continue
								}
								k := kerF[kerRow+kw]
								lo := sp.lo
								hi := min(sp.hi, a)
								if hi > lo {
									src := inF[inRow+lo+sp.tap : inRow+hi+sp.tap]
									dst := acc[lo:hi]
									for j, v := range src {
										dst[j] += v * k
									}
								}
								lo = max(sp.lo, b)
								hi = sp.hi
								if hi > lo {
									src := inF[inRow+lo+sp.tap : inRow+hi+sp.tap]
									dst := acc[lo:hi]
									for j, v := range src {
										dst[j] += v * k
									}
								}
							}
							if b <= a {
								continue
							}
							// The interior sweep. The taps
							// live in registers; the sub-slices
							// of one shifted window keep the
							// loop free of bounds checks and
							// index multiplies.
							src := inF[inRow+a-padW : inRow+b-padW+kW-1]
							dst := acc[a:b]
							n := len(dst)
							switch kW {
							case 2:
								k0, k1 := kerF[kerRow], kerF[kerRow+1]
								s1 := src[1 : 1+n]
								for i, v0 := range src[:n] {
									d := dst[i] + v0*k0
									dst[i] = d + s1[i]*k1
								}
							case 3:
								k0, k1, k2 := kerF[kerRow], kerF[kerRow+1], kerF[kerRow+2]
								s1 := src[1 : 1+n]
								s2 := src[2 : 2+n]
								for i, v0 := range src[:n] {
									d := dst[i] + v0*k0
									d += s1[i] * k1
									dst[i] = d + s2[i]*k2
								}
							case 4:
								k0, k1, k2, k3 := kerF[kerRow], kerF[kerRow+1], kerF[kerRow+2], kerF[kerRow+3]
								s1 := src[1 : 1+n]
								s2 := src[2 : 2+n]
								s3 := src[3 : 3+n]
								for i, v0 := range src[:n] {
									d := dst[i] + v0*k0
									d += s1[i] * k1
									d += s2[i] * k2
									dst[i] = d + s3[i]*k3
								}
							case 5:
								k0, k1, k2, k3, k4 := kerF[kerRow], kerF[kerRow+1], kerF[kerRow+2], kerF[kerRow+3], kerF[kerRow+4]
								s1 := src[1 : 1+n]
								s2 := src[2 : 2+n]
								s3 := src[3 : 3+n]
								s4 := src[4 : 4+n]
								for i, v0 := range src[:n] {
									d := dst[i] + v0*k0
									d += s1[i] * k1
									d += s2[i] * k2
									d += s3[i] * k3
									dst[i] = d + s4[i]*k4
								}
							case 6:
								k0, k1, k2, k3, k4, k5 := kerF[kerRow], kerF[kerRow+1], kerF[kerRow+2], kerF[kerRow+3], kerF[kerRow+4], kerF[kerRow+5]
								s1 := src[1 : 1+n]
								s2 := src[2 : 2+n]
								s3 := src[3 : 3+n]
								s4 := src[4 : 4+n]
								s5 := src[5 : 5+n]
								for i, v0 := range src[:n] {
									d := dst[i] + v0*k0
									d += s1[i] * k1
									d += s2[i] * k2
									d += s3[i] * k3
									d += s4[i] * k4
									dst[i] = d + s5[i]*k5
								}
							case 7:
								k0, k1, k2, k3, k4, k5, k6 := kerF[kerRow], kerF[kerRow+1], kerF[kerRow+2], kerF[kerRow+3], kerF[kerRow+4], kerF[kerRow+5], kerF[kerRow+6]
								s1 := src[1 : 1+n]
								s2 := src[2 : 2+n]
								s3 := src[3 : 3+n]
								s4 := src[4 : 4+n]
								s5 := src[5 : 5+n]
								s6 := src[6 : 6+n]
								for i, v0 := range src[:n] {
									d := dst[i] + v0*k0
									d += s1[i] * k1
									d += s2[i] * k2
									d += s3[i] * k3
									d += s4[i] * k4
									d += s5[i] * k5
									dst[i] = d + s6[i]*k6
								}
							}
							continue
						}
						for kw := range kW {
							sp := spans[kw]
							if !sp.valid {
								continue
							}
							k := kerF[kerRow+kw]
							for ow := sp.lo; ow < sp.hi; ow++ {
								acc[ow] += inF[inRow+ow*stride+sp.tap] * k
							}
						}
					}
				}
			}
			b := 0.0
			if biasF != nil {
				b = biasF[oc]
			}
			off := ((batch*cOut+oc)*dOut+od)*hOut*wOut + oh*wOut
			for ow := range wOut {
				outF[off+ow] = acc[ow] + b
			}
		}
	})
	return out, nil
}

// ConvTranspose2D performs the transposed convolution (sometimes
// called "deconvolution"). Input (N, C_in, H, W), kernel (C_in,
// C_out, kH, kW), output (N, C_out, H_out, W_out) where H_out =
// (H-1)*stride - 2*padding + kH (and analogously for W). bias is
// optional, applied per output channel.
func ConvTranspose2D(input, kernel, bias *core.Array, stride, padding int) (*core.Array, error) {
	if input.Dtype() == core.Complex || kernel.Dtype() == core.Complex {
		return nil, base.Errf("ConvTranspose2D: complex arrays are not supported")
	}
	if input.NDim() != 4 {
		return nil, base.Errf("ConvTranspose2D: input must be 4-D (N, C_in, H, W), got shape %s", base.ShapeText(input.Shape()))
	}
	if kernel.NDim() != 4 {
		return nil, base.Errf("ConvTranspose2D: kernel must be 4-D (C_in, C_out, kH, kW), got shape %s", base.ShapeText(kernel.Shape()))
	}
	if stride < 1 {
		return nil, base.Errf("ConvTranspose2D: stride must be at least 1, got %d", stride)
	}
	if padding < 0 {
		return nil, base.Errf("ConvTranspose2D: padding must be non-negative, got %d", padding)
	}
	n, cIn, hIn, wIn := input.Shape()[0], input.Shape()[1], input.Shape()[2], input.Shape()[3]
	cInK, cOut, kH, kW := kernel.Shape()[0], kernel.Shape()[1], kernel.Shape()[2], kernel.Shape()[3]
	if cInK != cIn {
		return nil, base.Errf("ConvTranspose2D: kernel in-channels %d does not match input %d", cInK, cIn)
	}
	hOut := (hIn-1)*stride - 2*padding + kH
	wOut := (wIn-1)*stride - 2*padding + kW
	if hOut < 1 || wOut < 1 {
		return nil, base.Errf("ConvTranspose2D: output is empty")
	}
	if bias != nil && (bias.NDim() != 1 || bias.Len() != cOut) {
		return nil, base.Errf("ConvTranspose2D: bias must be 1-D of length %d, got shape %s", cOut, base.ShapeText(bias.Shape()))
	}
	if bias != nil && bias.Dtype() == core.Complex {
		return nil, base.Errf("ConvTranspose2D: complex bias is not supported")
	}
	out, oerr := core.Zeros(core.Float, []int{n, cOut, hOut, wOut}...)
	if oerr != nil {
		return nil, oerr
	}
	inF := input.RawFloats()
	if inF == nil || input.Strided() {
		inF = widenFloats(input)
	}
	kerF := kernel.RawFloats()
	if kerF == nil || kernel.Strided() {
		kerF = widenFloats(kernel)
	}
	var biasF []float64
	if bias != nil {
		biasF = widenFloats(bias)
	}
	outF := out.RawFloats()
	// The scatter form (walking input positions and adding into every
	// output they touch) becomes a gather here: each output row sums
	// the input positions that map onto it. Blocked over output
	// channels, as Conv2D is: one work item is one (batch, output row,
	// channel block) and its accumulator holds every output channel of
	// the block at once, so a gathered input value is spent on every
	// channel of the block instead of being gathered again for each of
	// them. For a single output the contributions still arrive in the
	// (cIn, kH, kW) order the scatter visited them, and the bias is
	// added last, so the accumulated bits are unchanged. Rows are
	// disjoint, so the parallel split cannot touch a sum.
	//
	// The accumulator is partitioned by phase: a tap with reversed
	// column kw - padding only ever touches outputs congruent to it
	// modulo the stride, so each phase owns a contiguous sub-lattice
	// of every output row. A tap's gather then reads a contiguous
	// input window into a contiguous accumulator window, with no
	// divisions and no strided stores in the hot loop.
	ocBlock := max(1, min(cOut, convAccCap/wOut))
	blocksPerRow := (cOut + ocBlock - 1) / ocBlock
	rowCap := (wOut + stride - 1) / stride
	spans := convGatherSpans(kW, padding, stride, wIn, wOut)
	items := n * hOut * blocksPerRow
	engine.ParallelMin(items, workFloorFor(wOut*ocBlock*cIn*kH*kW), func(bs, be int) {
		acc := engine.GetFloat64Buf(ocBlock * stride * rowCap)
		defer engine.PutFloat64Buf(acc)
		var ihMap [16]int
		mapped := min(kH, len(ihMap))
		for item := bs; item < be; item++ {
			ob := item % blocksPerRow
			rest := item / blocksPerRow
			oh := rest % hOut
			batch := rest / hOut
			oc0 := ob * ocBlock
			oc1 := min(oc0+ocBlock, cOut)
			clear(acc)
			// Map (oh) back to the input rows once per item: the
			// scatter only paired positions with oh + padding - kh
			// divisible by the stride and inside the input, and the
			// mapping does not depend on the channel.
			for kh := range mapped {
				ih := -1
				if ihNum := oh + padding - kh; ihNum%stride == 0 {
					if t := ihNum / stride; t >= 0 && t < hIn {
						ih = t
					}
				}
				ihMap[kh] = ih
			}
			for ic := range cIn {
				inChan := batch*cIn*hIn*wIn + ic*hIn*wIn
				icBase := ic * cOut * kH * kW
				for kh := range kH {
					var ih int
					if kh < mapped {
						ih = ihMap[kh]
					} else {
						ihNum := oh + padding - kh
						if ihNum%stride != 0 {
							continue
						}
						ih = ihNum / stride
						if ih < 0 || ih >= hIn {
							continue
						}
					}
					if ih < 0 {
						continue
					}
					inRow := inChan + ih*wIn
					khOff := icBase + kh*kW
					for kw := range kW {
						sp := spans[kw]
						if !sp.valid {
							continue
						}
						tapOff := khOff + oc0*kH*kW + kw
						src := inF[inRow+sp.base : inRow+sp.base+sp.count]
						for o := range oc1 - oc0 {
							k := kerF[tapOff+o*kH*kW]
							off := (o*stride + sp.phase) * rowCap
							dst := acc[off+sp.j0 : off+sp.j0+sp.count]
							for j, v := range src {
								dst[j] += v * k
							}
						}
					}
				}
			}
			for o := range oc1 - oc0 {
				oc := oc0 + o
				b := 0.0
				if biasF != nil {
					b = biasF[oc]
				}
				off := (batch*cOut + oc) * hOut * wOut
				for p := range stride {
					phaseBase := (o*stride + p) * rowCap
					j := 0
					for ow := p; ow < wOut; ow += stride {
						outF[off+oh*wOut+ow] = acc[phaseBase+j] + b
						j++
					}
				}
			}
		}
	})
	return out, nil
}