package many_bits

import "base:builtin"
import "base:intrinsics"
import "core:fmt"
import "core:slice"

@(private)
ODIN_BOUNDS_CHECK :: !ODIN_NO_BOUNDS_CHECK
// Number of bits in system uint
UINT_NUM_BITS :: size_of(uint) * 8
UINT_MAX: uint : 1 << UINT_NUM_BITS - 1
// Power to which 2 is raised to get the size of uint in bits
// For bitshift division which gives index of integer in int_bits_array
INDEX_SHIFT :: uint(intrinsics.count_trailing_zeros(UINT_NUM_BITS))
// Value to & overall index by to get bit position
BIT_POS_MASK :: UINT_NUM_BITS - 1

Int_Bits :: bit_set[0 ..< UINT_NUM_BITS;uint]

// Use `core:container.Bit_Array` if dynamic length is needed.
// This has a more specific purpose.
Bits :: struct {
	int_array: []Int_Bits,
	length:    int, // Total number of bits being stored
}

delete :: proc(using bits: Bits, allocator := context.allocator) {
	delete_slice(int_array, allocator)
}

make :: proc(#any_int length: int, allocator := context.allocator) -> Bits {
	return Bits {
		int_array = make_slice([]Int_Bits, ((length - 1) >> INDEX_SHIFT) + 1, allocator),
		length = length,
	}
}

// Sets all bits to 0 (false)
zero :: #force_inline proc(bits: Bits) {
	slice.zero(bits.int_array)
}

set :: #force_inline proc(bits: Bits, #any_int index: int, set_to: bool) {
	when ODIN_BOUNDS_CHECK {
		if index >= bits.length {
			panic(fmt.tprintf("Bit position %i out of bounds for length %i.", index, bits.length))
		}
	}
	if set_to == true {
		bits.int_array[index >> INDEX_SHIFT] += {index & BIT_POS_MASK}
	} else {
		bits.int_array[index >> INDEX_SHIFT] -= {index & BIT_POS_MASK}
	}
}

set_true :: #force_inline proc(bits: Bits, #any_int index: int) {
	when ODIN_BOUNDS_CHECK {
		if index >= bits.length {
			panic(fmt.tprintf("Bit position %i out of bounds for length %i.", index, bits.length))
		}
	}
	bits.int_array[index >> INDEX_SHIFT] += {index & BIT_POS_MASK}
}

set_one :: set_true

set_false :: #force_inline proc(bits: Bits, #any_int index: int) {
	when ODIN_BOUNDS_CHECK {
		if index >= bits.length {
			panic(fmt.tprintf("Bit position %i out of bounds for length %i.", index, bits.length))
		}
	}
	bits.int_array[index >> INDEX_SHIFT] -= {index & BIT_POS_MASK}
}

set_zero :: set_false

get :: #force_inline proc(bits: Bits, #any_int index: int) -> bool {
	when ODIN_BOUNDS_CHECK {
		if index >= bits.length {
			panic(fmt.tprintf("Bit position %i out of bounds for length %i.", index, bits.length))
		}
	}
	return (index & BIT_POS_MASK) in bits.int_array[index >> INDEX_SHIFT]
}

// Returns true if all bits in range [start, end) are set [start is inclusive, end is exclusive)
range_true :: proc(bits: Bits, #any_int start, end: int) -> bool {
	when ODIN_BOUNDS_CHECK {
		if start < 0 {
			panic(fmt.tprintf("Start %i is negative.", start))
		}
		if start > end {
			panic(fmt.tprintf("Start %i is greater than end %i.", start, end))
		}
		if end > bits.length {
			panic(fmt.tprintf("End %i out of bounds for length %i.", end, bits.length))
		}
	}

	// Empty range is vacuously true
	if start == end do return true

	start_u := uint(start)
	end_u := uint(end)

	start_word := start_u >> INDEX_SHIFT
	end_word := (end_u - 1) >> INDEX_SHIFT

	start_bit := start_u & BIT_POS_MASK
	end_bit := end_u & BIT_POS_MASK // end is exclusive; 0 means "to end of word"

	// Range is within a single word
	if start_word == end_word {
		word := transmute(uint)bits.int_array[start_word]

		low_mask: uint = (uint(1) << start_bit) - 1
		high_mask: uint = ((uint(1) << end_bit) - 1) | (UINT_MAX * uint(end_bit == 0))

		mask := high_mask & ~low_mask
		return word & mask == mask
	}

	// Range spans multiple words

	// First word: [start_bit, UINT_NUM_BITS)
	if start_bit != 0 {
		first_word := transmute(uint)bits.int_array[start_word]
		start_mask: uint = ~((uint(1) << start_bit) - 1)
		if first_word & start_mask != start_mask {
			return false
		}
		start_word += 1
	}

	// Last word: [0, end_bit)
	// If end_bit == 0, we need the whole last word, so include it in the middle scan.
	if end_bit != 0 {
		last_word := transmute(uint)bits.int_array[end_word]
		end_mask: uint = (uint(1) << end_bit) - 1
		if last_word & end_mask != end_mask {
			return false
		}
	} else {
		end_word += 1
	}

	// Middle words: all bits must be set
	for i := start_word; i < end_word; i += 1 {
		if transmute(uint)bits.int_array[i] != UINT_MAX {
			return false
		}
	}

	return true
}

range_ones :: range_true

all_true :: proc(bits: Bits) -> bool {
	// Empty bit array is vacuously true
	if bits.length == 0 do return true

	bit_index := uint(bits.length - 1)
	int_index := bit_index >> INDEX_SHIFT
	// The last int needs special treatment because we only want to check part of it
	last_bit_pos := bit_index & BIT_POS_MASK
	last_bit_mask: uint = (1 << (last_bit_pos + 1)) - 1
	int_val := transmute(uint)bits.int_array[int_index]
	if int_val & last_bit_mask != last_bit_mask {
		return false
	}
	if int_index == 0 { 	// If there was only 1 int in the array
		return true
	}
	int_index -= 1

	// All other ints should be all 1s
	for {
		int_val := transmute(uint)bits.int_array[int_index]
		if int_val != UINT_MAX {
			return false
		}

		if int_index == 0 {
			return true
		}
		int_index -= 1
	}
}

all_ones :: all_true

// Returns ok = false if there are no 1 bits in the entire array.
nearest_true :: proc(bits: Bits, index: int) -> (nearest: int, ok: bool) {
	when ODIN_BOUNDS_CHECK {
		if index >= bits.length {
			panic(fmt.tprintf("Bit position %i out of bounds for length %i.", index, bits.length))
		}
	}

	bit_index := uint(index)
	word_index := bit_index >> INDEX_SHIFT
	bit_pos := bit_index & BIT_POS_MASK

	word_index_int := int(word_index)
	total_words := len(bits.int_array)
	max_left := word_index_int
	max_right := total_words - 1 - word_index_int
	max_offset := max(max_left, max_right)

	word_val := transmute(uint)bits.int_array[word_index_int]
	if word_val != 0 {
		if (word_val & (uint(1) << bit_pos)) != 0 do return index, true

		left_mask := (uint(1) << bit_pos) | ((uint(1) << bit_pos) - 1)
		left_bits_value := word_val & left_mask

		right_mask := ~((uint(1) << bit_pos) - 1)
		right_bits_value := word_val & right_mask

		nearest_left := 0
		left_found := false
		if left_bits_value != 0 {
			left_offset_from_top := intrinsics.count_leading_zeros(left_bits_value)
			left_bit := (UINT_NUM_BITS - 1) - left_offset_from_top
			nearest_left = (word_index_int << INDEX_SHIFT) + int(left_bit)
			left_found = true
		}

		nearest_right := 0
		right_found := false
		if right_bits_value != 0 {
			right_offset := intrinsics.count_trailing_zeros(right_bits_value)
			nearest_right = (word_index_int << INDEX_SHIFT) + int(right_offset)
			right_found = true
		}

		if left_found && right_found {
			left_dist := index - nearest_left
			right_dist := nearest_right - index
			if left_dist <= right_dist {
				return nearest_left, true
			} else {
				return nearest_right, true
			}
		} else if left_found {
			return nearest_left, true
		} else if right_found {
			return nearest_right, true
		}
	}

	for offset := 1; offset <= max_offset; offset += 1 {
		right_found := false
		left_found := false
		nearest_right := 0
		nearest_left := 0
		right_dist := 0
		left_dist := 0

		right_index := word_index_int + offset
		if right_index < total_words {
			word_val := transmute(uint)bits.int_array[right_index]
			if word_val != 0 {
				right_offset := intrinsics.count_trailing_zeros(word_val)
				nearest_right = (right_index << INDEX_SHIFT) + int(right_offset)
				right_found = true
				right_dist = nearest_right - index
			}
		}

		left_index := word_index_int - offset
		if left_index >= 0 {
			word_val := transmute(uint)bits.int_array[left_index]
			if word_val != 0 {
				left_offset_from_top := intrinsics.count_leading_zeros(word_val)
				left_bit := (UINT_NUM_BITS - 1) - left_offset_from_top
				nearest_left = (left_index << INDEX_SHIFT) + int(left_bit)
				left_found = true
				left_dist = index - nearest_left
			}
		}

		if left_found && right_found {
			if left_dist <= right_dist {
				return nearest_left, true
			} else {
				return nearest_right, true
			}
		} else if left_found {
			return nearest_left, true
		} else if right_found {
			return nearest_right, true
		}
	}

	return
}

nearest_one :: nearest_true

// Returns ok = false if there are no 0 bits in the entire array.
nearest_false :: proc(bits: Bits, index: int) -> (nearest: int, ok: bool) {
	when ODIN_BOUNDS_CHECK {
		if index >= bits.length {
			panic(fmt.tprintf("Bit position %i out of bounds for length %i.", index, bits.length))
		}
	}

	bit_index := uint(index)
	word_index := bit_index >> INDEX_SHIFT
	bit_pos := bit_index & BIT_POS_MASK

	word_index_int := int(word_index)
	total_words := len(bits.int_array)
	max_left := word_index_int
	max_right := total_words - 1 - word_index_int
	max_offset := max(max_left, max_right)

	last_bit_index := uint(bits.length - 1)
	last_word_index := int(last_bit_index >> INDEX_SHIFT)
	last_bit_pos := last_bit_index & BIT_POS_MASK
	valid_bits_mask: uint
	if last_bit_pos == UINT_NUM_BITS - 1 {
		valid_bits_mask = UINT_MAX
	} else {
		valid_bits_mask = (uint(1) << (last_bit_pos + 1)) - 1
	}

	word_val := transmute(uint)bits.int_array[word_index_int]
	word_val_search := ~word_val
	if word_index_int == last_word_index {
		word_val_search &= valid_bits_mask
	}
	if word_val_search != 0 {
		if (word_val & (uint(1) << bit_pos)) == 0 do return index, true

		left_mask := (uint(1) << bit_pos) | ((uint(1) << bit_pos) - 1)
		left_bits_value := word_val_search & left_mask

		right_mask := ~((uint(1) << bit_pos) - 1)
		right_bits_value := word_val_search & right_mask

		nearest_left := 0
		left_found := false
		if left_bits_value != 0 {
			left_offset_from_top := intrinsics.count_leading_zeros(left_bits_value)
			left_bit := (UINT_NUM_BITS - 1) - left_offset_from_top
			nearest_left = (word_index_int << INDEX_SHIFT) + int(left_bit)
			left_found = true
		}

		nearest_right := 0
		right_found := false
		if right_bits_value != 0 {
			right_offset := intrinsics.count_trailing_zeros(right_bits_value)
			nearest_right = (word_index_int << INDEX_SHIFT) + int(right_offset)
			right_found = true
		}

		if left_found && right_found {
			left_dist := index - nearest_left
			right_dist := nearest_right - index
			if left_dist <= right_dist {
				return nearest_left, true
			} else {
				return nearest_right, true
			}
		} else if left_found {
			return nearest_left, true
		} else if right_found {
			return nearest_right, true
		}
	}

	for offset := 1; offset <= max_offset; offset += 1 {
		right_found := false
		left_found := false
		nearest_right := 0
		nearest_left := 0
		right_dist := 0
		left_dist := 0

		right_index := word_index_int + offset
		if right_index < total_words {
			word_val := transmute(uint)bits.int_array[right_index]
			word_val_search := ~word_val
			if right_index == last_word_index {
				word_val_search &= valid_bits_mask
			}
			if word_val_search != 0 {
				right_offset := intrinsics.count_trailing_zeros(word_val_search)
				nearest_right = (right_index << INDEX_SHIFT) + int(right_offset)
				right_found = true
				right_dist = nearest_right - index
			}
		}

		left_index := word_index_int - offset
		if left_index >= 0 {
			word_val := transmute(uint)bits.int_array[left_index]
			word_val_search := ~word_val
			if word_val_search != 0 {
				left_offset_from_top := intrinsics.count_leading_zeros(word_val_search)
				left_bit := (UINT_NUM_BITS - 1) - left_offset_from_top
				nearest_left = (left_index << INDEX_SHIFT) + int(left_bit)
				left_found = true
				left_dist = index - nearest_left
			}
		}

		if left_found && right_found {
			if left_dist <= right_dist {
				return nearest_left, true
			} else {
				return nearest_right, true
			}
		} else if left_found {
			return nearest_left, true
		} else if right_found {
			return nearest_right, true
		}
	}

	return
}

nearest_zero :: nearest_false

Iterator :: struct {
	bits:     ^Bits,
	word_idx: int,
	bit_idx:  uint,
}

iterator :: #force_inline proc(bits: ^Bits) -> Iterator {
	return {bits = bits}
}

iterate :: proc(iterator: ^Iterator) -> (is_true: bool, idx: int, cond: bool) {
	idx = iterator.word_idx * UINT_NUM_BITS + int(iterator.bit_idx)
	if idx >= iterator.bits.length {
		return false, 0, false
	}

	word := transmute(uint)iterator.bits.int_array[iterator.word_idx]
	is_true = (word >> iterator.bit_idx & 1) == 1

	iterator.bit_idx += 1
	if iterator.bit_idx >= UINT_NUM_BITS {
		iterator.bit_idx = 0
		iterator.word_idx += 1
	}

	return is_true, idx, true
}

@(private = "file")
_iterate_kind :: #force_inline proc(iterator: ^Iterator, $ITERATE_ZEROS: bool) -> (idx: int, cond: bool) {
	for iterator.word_idx < len(iterator.bits.int_array) {
		word := transmute(uint)iterator.bits.int_array[iterator.word_idx]
		when ITERATE_ZEROS do word = ~word

		word >>= iterator.bit_idx // Mask out already-processed bits

		if word != 0 {
			// Found a bit - count_trailing_zeros gives position in shifted word
			iterator.bit_idx += uint(intrinsics.count_trailing_zeros(word))
			idx = iterator.word_idx * UINT_NUM_BITS + int(iterator.bit_idx)

			// Advance for next call
			iterator.bit_idx += 1
			if iterator.bit_idx >= UINT_NUM_BITS {
				iterator.bit_idx = 0
				iterator.word_idx += 1
			}

			return idx, idx < iterator.bits.length
		}

		// Word exhausted, move to next
		iterator.word_idx += 1
		iterator.bit_idx = 0
	}

	return 0, false
}

iterate_true :: proc(iterator: ^Iterator) -> (idx: int, cond: bool) {
	return _iterate_kind(iterator, ITERATE_ZEROS = false)
}

iterate_ones :: iterate_true

iterate_false :: proc(iterator: ^Iterator) -> (idx: int, cond: bool) {
	return _iterate_kind(iterator, ITERATE_ZEROS = true)
}

iterate_zeros :: iterate_false

// ---------------------------------------------------------------------------------------------------------------------
// ----- Tests ------------------------
// ---------------------------------------------------------------------------------------------------------------------
import "core:testing"

@(test)
test_set :: proc(t: ^testing.T) {
	bits := make(128)
	defer delete(bits)

	set(bits, 0, true)
	testing.expect_value(t, bits.int_array[0], Int_Bits{0})
	set(bits, 3, true)
	testing.expect_value(t, bits.int_array[0], Int_Bits{0, 3})
	set(bits, 64, true)
	testing.expect_value(t, bits.int_array[1], Int_Bits{0})
	set(bits, 127, true)
	testing.expect_value(t, bits.int_array[1], Int_Bits{0, 63})
	set(bits, 127, false)
	testing.expect_value(t, bits.int_array[1], Int_Bits{0})
}

@(test)
test_get :: proc(t: ^testing.T) {
	bits := make(128)
	defer delete(bits)

	// Default is false
	testing.expect(t, !get(bits, 0))
	testing.expect(t, !get(bits, 63))
	testing.expect(t, !get(bits, 64))
	testing.expect(t, !get(bits, 127))

	// Set and verify within first uint
	set(bits, 0, true)
	testing.expect(t, get(bits, 0))
	testing.expect(t, !get(bits, 1))

	set(bits, 3, true)
	testing.expect(t, get(bits, 3))
	testing.expect(t, !get(bits, 2))
	testing.expect(t, !get(bits, 4))

	// Cross uint boundary
	set(bits, 64, true)
	testing.expect(t, get(bits, 64))
	testing.expect(t, !get(bits, 63))
	testing.expect(t, !get(bits, 65))

	// Last bit
	set(bits, 127, true)
	testing.expect(t, get(bits, 127))

	// Unset and verify
	set(bits, 127, false)
	testing.expect(t, !get(bits, 127))
}

@(test)
test_set_true_set_false :: proc(t: ^testing.T) {
	bits := make(128)
	defer delete(bits)

	// set_true within first uint
	set_true(bits, 0)
	testing.expect_value(t, bits.int_array[0], Int_Bits{0})
	testing.expect(t, get(bits, 0))

	set_true(bits, 3)
	testing.expect_value(t, bits.int_array[0], Int_Bits{0, 3})
	testing.expect(t, get(bits, 3))

	// set_true across uint boundary
	set_true(bits, 64)
	testing.expect_value(t, bits.int_array[1], Int_Bits{0})
	testing.expect(t, get(bits, 64))
	testing.expect(t, !get(bits, 63))
	testing.expect(t, !get(bits, 65))

	// set_true on last bit
	set_true(bits, 127)
	testing.expect_value(t, bits.int_array[1], Int_Bits{0, 63})
	testing.expect(t, get(bits, 127))

	// set_false to clear bits
	set_false(bits, 127)
	testing.expect_value(t, bits.int_array[1], Int_Bits{0})
	testing.expect(t, !get(bits, 127))

	set_false(bits, 0)
	testing.expect_value(t, bits.int_array[0], Int_Bits{3})
	testing.expect(t, !get(bits, 0))
	testing.expect(t, get(bits, 3)) // bit 3 still set

	// set_false on already-false bit (should be no-op)
	set_false(bits, 1)
	testing.expect_value(t, bits.int_array[0], Int_Bits{3})
	testing.expect(t, !get(bits, 1))
}

@(test)
all_true_test :: proc(t: ^testing.T) {
	uint_max := UINT_MAX
	all_ones := transmute(Int_Bits)uint_max

	bits := make(132)
	defer delete(bits)

	bits.int_array[0] = all_ones
	bits.int_array[1] = all_ones
	bits.int_array[2] = {0, 1, 2, 3}
	testing.expect(t, all_true(bits))

	bits.int_array[2] = {0, 1, 2}
	testing.expect(t, !all_true(bits))

	bits2 := make(1)
	defer delete(bits2)

	bits2.int_array[0] = {0}
	testing.expect(t, all_true(bits2))
}

@(test)
test_range_true :: proc(t: ^testing.T) {
	uint_max := UINT_MAX
	all_ones := transmute(Int_Bits)uint_max

	bits := make(192)
	defer delete(bits)

	// Empty range is vacuously true
	testing.expect(t, range_true(bits, 0, 0))
	testing.expect(t, range_true(bits, 50, 50))
	// inverted range should panic under bounds checking; keep this test case out of here

	// Single word, partial range
	bits.int_array[0] = {3, 4, 5, 6}
	testing.expect(t, range_true(bits, 3, 7))
	testing.expect(t, !range_true(bits, 2, 7)) // bit 2 not set
	testing.expect(t, !range_true(bits, 3, 8)) // bit 7 not set

	// Single word, full word
	bits.int_array[0] = all_ones
	testing.expect(t, range_true(bits, 0, 64))

	// Range spanning two words
	bits.int_array[0] = all_ones
	bits.int_array[1] = {0, 1, 2, 3}
	testing.expect(t, range_true(bits, 60, 68)) // bits 60-63 in word 0, bits 0-3 in word 1
	testing.expect(t, !range_true(bits, 60, 69)) // bit 68 (4 in word 1) not set

	// Range spanning three words with full middle word
	bits.int_array[0] = all_ones
	bits.int_array[1] = all_ones
	bits.int_array[2] = {0, 1, 2, 3}
	testing.expect(t, range_true(bits, 60, 132)) // partial first, full middle, partial last
	testing.expect(t, !range_true(bits, 60, 133)) // bit 132 (4 in word 2) not set

	// Middle word not all set
	bits.int_array[1] = all_ones - {32}
	testing.expect(t, !range_true(bits, 60, 132))

	// Boundary: range ends exactly at word boundary
	bits.int_array[0] = all_ones
	bits.int_array[1] = all_ones
	testing.expect(t, range_true(bits, 32, 128))

	// Boundary: range starts exactly at word boundary
	bits.int_array[1] = all_ones
	bits.int_array[2] = all_ones
	testing.expect(t, range_true(bits, 64, 192))
}

@(test)
nearest_true_handles_same_word_and_boundaries :: proc(t: ^testing.T) {
	bits := make(128, context.temp_allocator)

	set_true(bits, 0)
	set_true(bits, 10)
	set_true(bits, 20)
	set_true(bits, 63)

	nearest, ok := nearest_true(bits, 10)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 10)

	nearest, ok = nearest_true(bits, 12)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 10)

	nearest, ok = nearest_true(bits, 17)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 20)

	nearest, ok = nearest_true(bits, 15)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 10)

	nearest, ok = nearest_true(bits, 0)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 0)

	nearest, ok = nearest_true(bits, 63)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 63)
}

@(test)
nearest_false_handles_same_word_and_boundaries :: proc(t: ^testing.T) {
	bits := make(128, context.temp_allocator)

	// Start with all bits true, then clear a few to false.
	for i := 0; i < bits.length; i += 1 {
		set_true(bits, i)
	}

	set_false(bits, 0)
	set_false(bits, 10)
	set_false(bits, 20)
	set_false(bits, 63)

	nearest, ok := nearest_false(bits, 10)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 10)

	nearest, ok = nearest_false(bits, 12)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 10)

	nearest, ok = nearest_false(bits, 17)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 20)

	nearest, ok = nearest_false(bits, 15)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 10)

	nearest, ok = nearest_false(bits, 0)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 0)

	nearest, ok = nearest_false(bits, 63)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 63)
}

@(test)
nearest_false_scans_across_words_and_returns_false_when_all_true :: proc(t: ^testing.T) {
	bits := make(192, context.temp_allocator)

	// Start with all bits true, then clear a couple far apart.
	for i := 0; i < bits.length; i += 1 {
		set_true(bits, i)
	}

	set_false(bits, 5)
	set_false(bits, 130)

	nearest, ok := nearest_false(bits, 96)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 130)

	// Restore the only zero bits so there are no zeros left.
	set_true(bits, 5)
	set_true(bits, 130)

	nearest, ok = nearest_false(bits, 96)
	testing.expect(t, !ok)
}

@(test)
nearest_true_scans_across_words_and_returns_false_when_empty :: proc(t: ^testing.T) {
	bits := make(192, context.temp_allocator)

	set_true(bits, 5)
	set_true(bits, 130)

	nearest, ok := nearest_true(bits, 96)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 130)

	zero(bits)

	nearest, ok = nearest_true(bits, 96)
	testing.expect(t, !ok)
}

@(test)
nearest_false_handles_last_word_partial_length :: proc(t: ^testing.T) {
	bits := make(130, context.temp_allocator)

	// Start with all bits true, then clear the first and last valid bits.
	for i := 0; i < bits.length; i += 1 {
		set_true(bits, i)
	}

	set_false(bits, 0)
	set_false(bits, 129)

	nearest, ok := nearest_false(bits, 128)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 129)

	nearest, ok = nearest_false(bits, 127)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 129)
}

@(test)
nearest_true_handles_last_word_partial_length :: proc(t: ^testing.T) {
	bits := make(130, context.temp_allocator)

	set_true(bits, 0)
	set_true(bits, 129)

	nearest, ok := nearest_true(bits, 128)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 129)

	nearest, ok = nearest_true(bits, 127)
	testing.expect(t, ok)
	testing.expect_value(t, nearest, 129)
}

@(test)
iterator_basic_mixed_bits :: proc(t: ^testing.T) {
	// Use non-word-aligned length to test partial last word handling
	bits := make(100, context.temp_allocator)

	// Set specific bits: 0, 3, 64, 99 (last valid index)
	set_true(bits, 0)
	set_true(bits, 3)
	set_true(bits, 64)
	set_true(bits, 99)

	expected_true_indices := [?]int{0, 3, 64, 99}

	// Test iterate - should return all 100 bits with correct set state
	{
		it := iterator(&bits)
		count := 0
		for is_set, idx in iterate(&it) {
			expected_set := slice.contains(expected_true_indices[:], idx)
			testing.expectf(
				t,
				is_set == expected_set,
				"iterate: bit %d expected is_set=%v, got %v",
				idx,
				expected_set,
				is_set,
			)
			testing.expectf(t, idx == count, "iterate: expected sequential idx=%d, got %d", count, idx)
			count += 1
		}
		testing.expectf(t, count == 100, "iterate: expected 100 iterations, got %d", count)
	}

	// Test iterate_true - should only return the 4 set bits
	{
		it := iterator(&bits)
		result_indices := builtin.make([dynamic]int, allocator = context.temp_allocator)
		for idx in iterate_true(&it) {
			append(&result_indices, idx)
		}
		testing.expectf(
			t,
			len(result_indices) == 4,
			"iterate_true: expected 4 set bits, got %d",
			len(result_indices),
		)
		for expected_idx, i in expected_true_indices {
			testing.expectf(
				t,
				result_indices[i] == expected_idx,
				"iterate_true: at position %d expected idx=%d, got %d",
				i,
				expected_idx,
				result_indices[i],
			)
		}
	}

	// Test iterate_false - should return all 96 unset bits
	{
		it := iterator(&bits)
		count := 0
		for idx in iterate_false(&it) {
			testing.expectf(
				t,
				!slice.contains(expected_true_indices[:], idx),
				"iterate_false: returned set bit index %d",
				idx,
			)
			count += 1
		}
		testing.expectf(t, count == 96, "iterate_false: expected 96 unset bits, got %d", count)
	}
}

@(test)
iterator_all_false_bits :: proc(t: ^testing.T) {
	// Use non-word-aligned length
	bits := make(100, context.temp_allocator)
	// All bits default to false, no need to set anything

	// Test iterate - should return all 100 bits as false
	{
		it := iterator(&bits)
		count := 0
		for is_set, idx in iterate(&it) {
			testing.expectf(t, !is_set, "iterate: bit %d expected is_set=false, got true", idx)
			testing.expectf(t, idx == count, "iterate: expected sequential idx=%d, got %d", count, idx)
			count += 1
		}
		testing.expectf(t, count == 100, "iterate: expected 100 iterations, got %d", count)
	}

	// Test iterate_true - should return nothing
	{
		it := iterator(&bits)
		count := 0
		for idx in iterate_true(&it) {
			testing.expectf(t, false, "iterate_true: unexpectedly returned idx=%d when all bits are false", idx)
			count += 1
		}
		testing.expectf(t, count == 0, "iterate_true: expected 0 iterations, got %d", count)
	}

	// Test iterate_false - should return all 100 indices
	{
		it := iterator(&bits)
		count := 0
		for idx in iterate_false(&it) {
			testing.expectf(t, idx == count, "iterate_false: expected sequential idx=%d, got %d", count, idx)
			count += 1
		}
		testing.expectf(t, count == 100, "iterate_false: expected 100 iterations, got %d", count)
	}
}

@(test)
iterator_all_true_bits :: proc(t: ^testing.T) {
	// Use non-word-aligned length
	bits := make(100, context.temp_allocator)
	// Set all bits to true
	for i := 0; i < bits.length; i += 1 {
		set_true(bits, i)
	}

	// Test iterate - should return all 100 bits as true
	{
		it := iterator(&bits)
		count := 0
		for is_set, idx in iterate(&it) {
			testing.expectf(t, is_set, "iterate: bit %d expected is_set=true, got false", idx)
			testing.expectf(t, idx == count, "iterate: expected sequential idx=%d, got %d", count, idx)
			count += 1
		}
		testing.expectf(t, count == 100, "iterate: expected 100 iterations, got %d", count)
	}

	// Test iterate_true - should return all 100 indices
	{
		it := iterator(&bits)
		count := 0
		for idx in iterate_true(&it) {
			testing.expectf(t, idx == count, "iterate_true: expected sequential idx=%d, got %d", count, idx)
			count += 1
		}
		testing.expectf(t, count == 100, "iterate_true: expected 100 iterations, got %d", count)
	}

	// Test iterate_false - should return nothing
	{
		it := iterator(&bits)
		count := 0
		for idx in iterate_false(&it) {
			testing.expectf(t, false, "iterate_false: unexpectedly returned idx=%d when all bits are true", idx)
			count += 1
		}
		testing.expectf(t, count == 0, "iterate_false: expected 0 iterations, got %d", count)
	}
}