package arena

import (
	"errors"
	"log"
	"math/bits"
	"runtime"
	"sync"
	"unsafe"

	"git.brut.systems/judah/xx/mem"
)

// Linear is a simple bump allocator with a fixed amount of backing memory.
func Linear(capacity_in_bytes uintptr) Arena {
	if capacity_in_bytes <= 0 {
		panic("linear: capacity_in_bytes must be greater than zero")
	}

	var (
		data   = make([]byte, capacity_in_bytes)
		offset uintptr
	)
	return func(a Action, size, align uintptr, watermark *uintptr) (unsafe.Pointer, error) {
		switch a {
		case ACTION_ALLOC:
			aligned := mem.AlignForward(size, align)
			if offset+aligned > capacity_in_bytes {
				return nil, errors.New("linear: out of memory")
			}

			ptr := &data[offset]
			offset += aligned
			return unsafe.Pointer(ptr), nil
		case ACTION_RESET:
			clear(data)
			offset = 0
		case ACTION_SAVE:
			if watermark == nil {
				return nil, errors.New("linear: cannot save to nil watermark")
			}

			*watermark = offset
		case ACTION_RESTORE:
			if watermark == nil {
				return nil, errors.New("linear: cannot restore nil watermark")
			}

			clear(data[*watermark:offset])
			offset = *watermark
		default:
			panic("linear: unimplemented action - " + a.String())
		}

		return nil, nil
	}
}

// Ring is an Arena that only allocates values of the given type.
// When capacity is exceeded, previous allocations will be reused to accommodate new ones
//
// Note: Allocating different types from the same Pool is unsafe and may cause memory corruption.
func Ring[T any](capacity uintptr) Arena {
	if capacity <= 0 {
		panic("pool: capacity must be greater than zero")
	}

	pointers := make([]T, 0, capacity)
	return func(a Action, _, _ uintptr, watermark *uintptr) (unsafe.Pointer, error) {
		switch a {
		case ACTION_ALLOC:
			if len(pointers) == cap(pointers) {
				pointers = pointers[:0]
			}

			pointers = append(pointers, mem.ZeroValue[T]())
			return unsafe.Pointer(&pointers[len(pointers)-1]), nil
		case ACTION_RESET:
			clear(pointers)
			pointers = pointers[:0]
		case ACTION_SAVE:
			if watermark == nil {
				return nil, errors.New("pool: cannot save to nil watermark")
			}

			*watermark = uintptr(len(pointers))
		case ACTION_RESTORE:
			if watermark == nil {
				return nil, errors.New("pool: cannot restore nil watermark")
			}

			clear(pointers[*watermark:])
			pointers = pointers[:*watermark]
		default:
			panic("pool: unimplemented action - " + a.String())
		}

		return nil, nil
	}
}

// Chunked is an Arena that groups allocations by size.
func Chunked(max_allocs_per_chunk uintptr) Arena {
	type chunk struct {
		data   []byte
		offset uintptr
		saved  uintptr
	}

	groups := make([][]chunk, 64)
	return func(a Action, size, align uintptr, _ *uintptr) (unsafe.Pointer, error) {
		switch a {
		case ACTION_ALLOC:
			aligned := mem.AlignForward(size, align)
			if aligned == 0 {
				aligned = 1
			}
			aligned = 1 << bits.Len(uint(aligned-1))

			idx := bits.TrailingZeros(uint(aligned))
			if idx >= len(groups) {
				groups = append(groups, make([][]chunk, idx-len(groups)+1)...)
			}

			group := groups[idx]
			if len(group) == 0 {
				group = append(group, chunk{
					data: make([]byte, aligned*max_allocs_per_chunk),
				})
			}

			c := &group[len(group)-1]
			if c.offset+aligned > uintptr(len(c.data)) {
				group = append(group, chunk{
					data: make([]byte, aligned*max_allocs_per_chunk),
				})

				c = &group[len(group)-1]
			}

			ptr := &c.data[c.offset]
			c.offset += aligned
			groups[idx] = group

			return unsafe.Pointer(ptr), nil
		case ACTION_RESET:
			for _, g := range groups {
				for i := range len(g) {
					g[i].offset = 0
					g[i].saved = 0
					clear(g[i].data)
				}
			}
		case ACTION_SAVE:
			for _, g := range groups {
				for i := range len(g) {
					g[i].saved = g[i].offset
				}
			}
		case ACTION_RESTORE:
			for _, g := range groups {
				for i := range len(g) {
					g[i].offset = g[i].saved
				}
			}
		default:
			panic("chunked: unimplemented action - " + a.String())
		}

		return nil, nil
	}
}

// Nil is an Arena that always returns an error.
//
// Note: This is useful for tracking usage locations
func Nil() Arena {
	return func(a Action, size, align uintptr, watermark *uintptr) (unsafe.Pointer, error) {
		return nil, errors.New("use of nil allocator")
	}
}

// Region wraps an Arena, restoring it to its previous state when Reset is called.
func Region(arena Arena) Arena {
	watermark := Save(arena)
	return func(a Action, size, align uintptr, wm *uintptr) (unsafe.Pointer, error) {
		if a == ACTION_RESET {
			Restore(arena, watermark)
			return nil, nil
		}

		return arena(a, size, align, wm)
	}
}

// Split wraps two [[Arena]]s, dispatching allocations to a particular one based on the requested size.
func Split(split_size uintptr, smaller, larger Arena) Arena {
	var watermarks [2]uintptr
	return func(a Action, size, align uintptr, watermark *uintptr) (unsafe.Pointer, error) {
		switch a {
		case ACTION_ALLOC:
			if size <= split_size {
				return smaller(a, size, align, watermark)
			}
			return larger(a, size, align, watermark)
		case ACTION_RESET:
			Reset(smaller)
			Reset(larger)
		case ACTION_SAVE:
			watermarks[0] = Save(smaller)
			watermarks[1] = Save(larger)
		case ACTION_RESTORE:
			Restore(smaller, watermarks[0])
			Restore(larger, watermarks[1])
		default:
			panic("split: unimplemented action - " + a.String())
		}

		return nil, nil
	}
}

// Logger wraps an Arena, logging its usage locations.
func Logger(arena Arena) Arena {
	return func(a Action, size, align uintptr, watermark *uintptr) (unsafe.Pointer, error) {
		// We expect allocators to be used via the high-level API, so we grab the caller location relative to that.
		// @todo(judah): can we determine this dynamically?
		_, file, line, ok := runtime.Caller(2)
		if !ok {
			file = "<unknown>"
			line = 0
		}

		log.Printf("%s:%d - %s (size: %d, align: %d, watermark: %p)", file, line, a, size, align, watermark)
		return arena(a, size, align, watermark)
	}
}

// Concurrent wraps an Arena, ensuring it is safe for concurrent use.
func Concurrent(arena Arena) Arena {
	mtx := new(sync.Mutex)
	return func(a Action, size, align uintptr, watermark *uintptr) (unsafe.Pointer, error) {
		mtx.Lock()
		ptr, err := arena(a, size, align, watermark)
		mtx.Unlock()
		return ptr, err
	}
}

// Pinned wraps an Arena, ensuring the memory returned is stable until Reset is called.
//
// The memory returned by Pinned is safe to pass over cgo boundaries.
func Pinned(arena Arena) Arena {
	var pinner runtime.Pinner
	return func(a Action, size, align uintptr, watermark *uintptr) (unsafe.Pointer, error) {
		ptr, err := arena(a, size, align, watermark)
		if err != nil {
			return ptr, err
		}

		if a == ACTION_RESET {
			pinner.Unpin()
		} else {
			pinner.Pin(ptr)
		}

		return ptr, err
	}
}