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

package spmd

import "sync"

// The routed frame pool recycles the payload buffers of the frames the
// hub routes on, the frames whose destination is another rank. Its
// safety rests on one ownership chain and nothing else: the pump takes
// a buffer when it reads a routed frame, the frame hands the buffer to
// the destination link's outbox channel, and the drain, the chain's
// single consumer, writes the frame and returns the buffer at once,
// on a landed write and on a benign skip at a departing peer alike.
// No frame travels a pending stash with the pool's mark on, no
// collective ever sees a pooled buffer, and no count, atom or second
// return point exists: a channel handoff is the ownership transfer.
// A buffer the chain drops on the way, a mislabelled frame or a world
// that ends mid-route, is left to the collector, which makes the loss
// a pool miss and never a double return. This single owner is why the
// first pool attempt's failure mode, a payload retired while a decode
// still read it, cannot arise here.
//
// Two bounds keep the pool from pinning memory. A buffer above the
// retention ceiling is never kept, so one enormous frame cannot pin
// the heap across rounds, and the pool holds at most a fixed number
// of buffers, so a flood of mid-sized ones cannot grow without end.
// Both refusals are pool misses: the buffer goes to the collector,
// which is exactly the behaviour the package had before the pool.

const (
	// framePoolCeiling is the largest buffer the pool retains, one
	// mebibyte. A payload beyond it is a pool miss by rule.
	framePoolCeiling = int64(1 << 20)
	// framePoolBound is the largest number of buffers the pool holds
	// at once; a return that would exceed it is dropped.
	framePoolBound = 32
)

// framePoolEnabled switches the pool at package level. It exists for
// the A/B measurement, which flips it between alternating
// sub-benchmarks of one binary; the library itself always runs pooled.
var framePoolEnabled = true

// framePool is the free list behind the pool: payload buffers waiting
// for the next routed frame whose length fits.
type framePool struct {
	mu   sync.Mutex
	free [][]byte
}

// routedFrames is the process's one pool. The worlds of one process
// share it, which is what makes its bounds process-wide facts.
var routedFrames framePool

// takeFrameBuffer answers the buffer a routed frame's payload reads
// into, or nil when the frame allocates as usual, outside the pool's
// chain: when the pool is switched off, when the frame stays at the
// hub, or when the length the header announced is beyond the ceiling.
// A qualified frame rides the chain whatever the free list holds: the
// buffer comes from the pool when one fits, and a fresh one joins the
// chain in its place, so the drain has a buffer to return and the
// pool warms with the traffic it serves. The link read calls this
// with the header already parsed, before the payload moves.
func takeFrameBuffer(m message, length int64) []byte {
	if !framePoolEnabled || m.dest == 0 || length <= 0 || length > framePoolCeiling {
		return nil
	}
	if buf := routedFrames.take(length); buf != nil {
		return buf
	}
	return make([]byte, length)
}

// take answers a buffer of exactly length bytes with at least that
// capacity, the smallest retained buffer that fits, or nil when the
// pool holds none.
func (p *framePool) take(length int64) []byte {
	if length <= 0 || length > framePoolCeiling {
		return nil
	}
	p.mu.Lock()
	defer p.mu.Unlock()
	best := -1
	for i, buf := range p.free {
		if int64(cap(buf)) >= length && (best < 0 || cap(buf) < cap(p.free[best])) {
			best = i
		}
	}
	if best < 0 {
		return nil
	}
	buf := p.free[best]
	p.free = append(p.free[:best], p.free[best+1:]...)
	return buf[:length]
}

// retire returns one buffer to the pool under the two bounds: a
// buffer whose capacity exceeds the ceiling, and a return that would
// push the pool past its count bound, are dropped to the collector.
func (p *framePool) retire(data []byte) {
	if !framePoolEnabled || cap(data) == 0 || int64(cap(data)) > framePoolCeiling {
		return
	}
	p.mu.Lock()
	if len(p.free) < framePoolBound {
		p.free = append(p.free, data[:cap(data)])
	}
	p.mu.Unlock()
}