// Copyright (c) 2019-2024 Alexander Medvednikov. All rights reserved.
// Use of this source code is governed by an MIT license
// that can be found in the LICENSE file.
module bits

// See http://supertech.csail.mit.edu/papers/debruijn.pdf
const de_bruijn32 = u32(0x077CB531)
const de_bruijn32tab = [u8(0), 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8, 31, 27, 13,
    23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9]!
const de_bruijn64 = u64(0x03f79d71b4ca8b09)
const de_bruijn64tab = [u8(0), 1, 56, 2, 57, 49, 28, 3, 61, 58, 42, 50, 38, 29, 17, 4, 62, 47,
    59, 36, 45, 43, 51, 22, 53, 39, 33, 30, 24, 18, 12, 5, 63, 55, 48, 27, 60, 41, 37, 16, 46,
    35, 44, 21, 52, 32, 23, 11, 54, 26, 40, 15, 34, 20, 31, 10, 25, 14, 19, 9, 13, 8, 7, 6]!

const m0 = u64(0x5555555555555555) // 01010101 ...

const m1 = u64(0x3333333333333333) // 00110011 ...

const m2 = u64(0x0f0f0f0f0f0f0f0f) // 00001111 ...

const m3 = u64(0x00ff00ff00ff00ff) // etc.

const m4 = u64(0x0000ffff0000ffff)

// --- LeadingZeros ---
// leading_zeros_8 returns the number of leading zero bits in x; the result is 8 for x == 0.
@[inline]
pub fn leading_zeros_8(x u8) int {
    return leading_zeros_8_default(x)
}

@[inline]
fn leading_zeros_8_default(x u8) int {
    return 8 - len_8(x)
}

// leading_zeros_16 returns the number of leading zero bits in x; the result is 16 for x == 0.
@[inline]
pub fn leading_zeros_16(x u16) int {
    return leading_zeros_16_default(x)
}

@[inline]
fn leading_zeros_16_default(x u16) int {
    return 16 - len_16(x)
}

// leading_zeros_32 returns the number of leading zero bits in x; the result is 32 for x == 0.
@[inline]
pub fn leading_zeros_32(x u32) int {
    return leading_zeros_32_default(x)
}

@[inline]
fn leading_zeros_32_default(x u32) int {
    return 32 - len_32(x)
}

// leading_zeros_64 returns the number of leading zero bits in x; the result is 64 for x == 0.
@[inline]
pub fn leading_zeros_64(x u64) int {
    return leading_zeros_64_default(x)
}

@[inline]
fn leading_zeros_64_default(x u64) int {
    return 64 - len_64(x)
}

// --- TrailingZeros ---
// trailing_zeros_8 returns the number of trailing zero bits in x; the result is 8 for x == 0.
@[inline]
pub fn trailing_zeros_8(x u8) int {
    return trailing_zeros_8_default(x)
}

@[direct_array_access; inline]
fn trailing_zeros_8_default(x u8) int {
    return int(ntz_8_tab[x])
}

// trailing_zeros_16 returns the number of trailing zero bits in x; the result is 16 for x == 0.
@[inline]
pub fn trailing_zeros_16(x u16) int {
    return trailing_zeros_16_default(x)
}

@[direct_array_access; ignore_overflow; inline]
fn trailing_zeros_16_default(x u16) int {
    if x == 0 {
        return 16
    }
    // see comment in trailing_zeros_64
    return int(de_bruijn32tab[u32(x & -x) * de_bruijn32 >> (32 - 5)])
}

// trailing_zeros_32 returns the number of trailing zero bits in x; the result is 32 for x == 0.
@[inline]
pub fn trailing_zeros_32(x u32) int {
    return trailing_zeros_32_default(x)
}

@[direct_array_access; ignore_overflow; inline]
fn trailing_zeros_32_default(x u32) int {
    if x == 0 {
        return 32
    }
    // see comment in trailing_zeros_64
    return int(de_bruijn32tab[(x & -x) * de_bruijn32 >> (32 - 5)])
}

// trailing_zeros_64 returns the number of trailing zero bits in x; the result is 64 for x == 0.
@[inline]
pub fn trailing_zeros_64(x u64) int {
    return trailing_zeros_64_default(x)
}

@[direct_array_access; ignore_overflow; inline]
fn trailing_zeros_64_default(x u64) int {
    if x == 0 {
        return 64
    }
    // If popcount is fast, replace code below with return popcount(^x & (x - 1)).
    //
    // x & -x leaves only the right-most bit set in the word. Let k be the
    // index of that bit. Since only a single bit is set, the value is two
    // to the power of k. Multiplying by a power of two is equivalent to
    // left shifting, in this case by k bits. The de Bruijn (64 bit) constant
    // is such that all six bit, consecutive substrings are distinct.
    // Therefore, if we have a left shifted version of this constant we can
    // find by how many bits it was shifted by looking at which six bit
    // substring ended up at the top of the word.
    // (Knuth, volume 4, section 7.3.1)
    return int(de_bruijn64tab[int((x & -x) * de_bruijn64 >> (64 - 6))])
}

// --- OnesCount ---
// ones_count_8 returns the number of one bits ("population count") in x.
@[inline]
pub fn ones_count_8(x u8) int {
    return ones_count_8_default(x)
}

@[direct_array_access; inline]
fn ones_count_8_default(x u8) int {
    return int(pop_8_tab[x])
}

// ones_count_16 returns the number of one bits ("population count") in x.
@[inline]
pub fn ones_count_16(x u16) int {
    return ones_count_16_default(x)
}

@[direct_array_access; inline]
fn ones_count_16_default(x u16) int {
    return int(pop_8_tab[x >> 8] + pop_8_tab[x & u16(0xff)])
}

// ones_count_32 returns the number of one bits ("population count") in x.
@[inline]
pub fn ones_count_32(x u32) int {
    return ones_count_32_default(x)
}

@[direct_array_access; inline]
fn ones_count_32_default(x u32) int {
    return int(pop_8_tab[x >> 24] + pop_8_tab[(x >> 16) & 0xff] + pop_8_tab[(x >> 8) & 0xff] +
        pop_8_tab[x & u32(0xff)])
}

// ones_count_64 returns the number of one bits ("population count") in x.
@[inline]
pub fn ones_count_64(x u64) int {
    return ones_count_64_default(x)
}

fn ones_count_64_default(x u64) int {
    // Implementation: Parallel summing of adjacent bits.
    // See "Hacker's Delight", Chap. 5: Counting Bits.
    // The following pattern shows the general approach:
    //
    // x = x>>1&(m0&m) + x&(m0&m)
    // x = x>>2&(m1&m) + x&(m1&m)
    // x = x>>4&(m2&m) + x&(m2&m)
    // x = x>>8&(m3&m) + x&(m3&m)
    // x = x>>16&(m4&m) + x&(m4&m)
    // x = x>>32&(m5&m) + x&(m5&m)
    // return int(x)
    //
    // Masking (& operations) can be left away when there's no
    // danger that a field's sum will carry over into the next
    // field: Since the result cannot be > 64, 8 bits is enough
    // and we can ignore the masks for the shifts by 8 and up.
    // Per "Hacker's Delight", the first line can be simplified
    // more, but it saves at best one instruction, so we leave
    // it alone for clarity.
    mut y := ((x >> u64(1)) & (m0 & max_u64)) + (x & (m0 & max_u64))
    y = ((y >> u64(2)) & (m1 & max_u64)) + (y & (m1 & max_u64))
    y = ((y >> 4) + y) & (m2 & max_u64)
    y += y >> 8
    y += y >> 16
    y += y >> 32
    return int(y) & ((1 << 7) - 1)
}

const n8 = u8(8)
const n16 = u16(16)
const n32 = u32(32)
const n64 = u64(64)

// --- RotateLeft ---
// rotate_left_8 returns the value of x rotated left by (k mod 8) bits.
// To rotate x right by k bits, call rotate_left_8(x, -k).
//
// This function's execution time does not depend on the inputs.
@[inline]
pub fn rotate_left_8(x u8, k int) u8 {
    s := u8(k) & (n8 - u8(1))
    return (x << s) | (x >> (n8 - s))
}

// rotate_left_16 returns the value of x rotated left by (k mod 16) bits.
// To rotate x right by k bits, call rotate_left_16(x, -k).
//
// This function's execution time does not depend on the inputs.
@[inline]
pub fn rotate_left_16(x u16, k int) u16 {
    s := u16(k) & (n16 - u16(1))
    return (x << s) | (x >> (n16 - s))
}

// rotate_left_32 returns the value of x rotated left by (k mod 32) bits.
// To rotate x right by k bits, call rotate_left_32(x, -k).
//
// This function's execution time does not depend on the inputs.
@[inline]
pub fn rotate_left_32(x u32, k int) u32 {
    s := u32(k) & (n32 - u32(1))
    return (x << s) | (x >> (n32 - s))
}

// rotate_left_64 returns the value of x rotated left by (k mod 64) bits.
// To rotate x right by k bits, call rotate_left_64(x, -k).
//
// This function's execution time does not depend on the inputs.
@[inline]
pub fn rotate_left_64(x u64, k int) u64 {
    s := u64(k) & (n64 - u64(1))
    return (x << s) | (x >> (n64 - s))
}

// --- Reverse ---
// reverse_8 returns the value of x with its bits in reversed order.
@[direct_array_access; inline]
pub fn reverse_8(x u8) u8 {
    return rev_8_tab[x]
}

// reverse_16 returns the value of x with its bits in reversed order.
@[direct_array_access; inline]
pub fn reverse_16(x u16) u16 {
    return u16(rev_8_tab[x >> 8]) | (u16(rev_8_tab[x & u16(0xff)]) << 8)
}

// reverse_32 returns the value of x with its bits in reversed order.
@[inline]
pub fn reverse_32(x u32) u32 {
    mut y := (((x >> u32(1)) & (m0 & max_u32)) | ((x & (m0 & max_u32)) << 1))
    y = (((y >> u32(2)) & (m1 & max_u32)) | ((y & (m1 & max_u32)) << u32(2)))
    y = (((y >> u32(4)) & (m2 & max_u32)) | ((y & (m2 & max_u32)) << u32(4)))
    return reverse_bytes_32(u32(y))
}

// reverse_64 returns the value of x with its bits in reversed order.
@[inline]
pub fn reverse_64(x u64) u64 {
    mut y := (((x >> u64(1)) & (m0 & max_u64)) | ((x & (m0 & max_u64)) << 1))
    y = (((y >> u64(2)) & (m1 & max_u64)) | ((y & (m1 & max_u64)) << 2))
    y = (((y >> u64(4)) & (m2 & max_u64)) | ((y & (m2 & max_u64)) << 4))
    return reverse_bytes_64(y)
}

// --- ReverseBytes ---
// reverse_bytes_16 returns the value of x with its bytes in reversed order.
//
// This function's execution time does not depend on the inputs.
@[inline]
pub fn reverse_bytes_16(x u16) u16 {
    return (x >> 8) | (x << 8)
}

// reverse_bytes_32 returns the value of x with its bytes in reversed order.
//
// This function's execution time does not depend on the inputs.
@[inline]
pub fn reverse_bytes_32(x u32) u32 {
    y := (((x >> u32(8)) & (m3 & max_u32)) | ((x & (m3 & max_u32)) << u32(8)))
    return u32((y >> 16) | (y << 16))
}

// reverse_bytes_64 returns the value of x with its bytes in reversed order.
//
// This function's execution time does not depend on the inputs.
@[inline]
pub fn reverse_bytes_64(x u64) u64 {
    mut y := (((x >> u64(8)) & (m3 & max_u64)) | ((x & (m3 & max_u64)) << u64(8)))
    y = (((y >> u64(16)) & (m4 & max_u64)) | ((y & (m4 & max_u64)) << u64(16)))
    return (y >> 32) | (y << 32)
}

// --- Len ---
// len_8 returns the minimum number of bits required to represent x; the result is 0 for x == 0.
@[direct_array_access]
pub fn len_8(x u8) int {
    return int(len_8_tab[x])
}

// len_16 returns the minimum number of bits required to represent x; the result is 0 for x == 0.
@[direct_array_access; ignore_overflow]
pub fn len_16(x u16) int {
    mut y := x
    mut n := 0
    if y >= 1 << 8 {
        y >>= 8
        n = 8
    }
    return n + int(len_8_tab[int(y)])
}

// len_32 returns the minimum number of bits required to represent x; the result is 0 for x == 0.
@[direct_array_access; ignore_overflow]
pub fn len_32(x u32) int {
    mut y := x
    mut n := 0
    if y >= (1 << 16) {
        y >>= 16
        n = 16
    }
    if y >= (1 << 8) {
        y >>= 8
        n += 8
    }
    return n + int(len_8_tab[int(y)])
}

// len_64 returns the minimum number of bits required to represent x; the result is 0 for x == 0.
@[direct_array_access]
pub fn len_64(x u64) int {
    mut y := x
    mut n := 0
    if y >= u64(1) << u64(32) {
        y >>= 32
        n = 32
    }
    if y >= u64(1) << u64(16) {
        y >>= 16
        n += 16
    }
    if y >= u64(1) << u64(8) {
        y >>= 8
        n += 8
    }
    return n + int(len_8_tab[int(y)])
}

// --- Add with carry ---
// Add returns the sum with carry of x, y and carry: sum = x + y + carry.
// The carry input must be 0 or 1; otherwise the behavior is undefined.
// The carryOut output is guaranteed to be 0 or 1.
// add_32 returns the sum with carry of x, y and carry: sum = x + y + carry.
// The carry input must be 0 or 1; otherwise the behavior is undefined.
// The carryOut output is guaranteed to be 0 or 1.
// This function's execution time does not depend on the inputs.
@[ignore_overflow]
pub fn add_32(x u32, y u32, carry u32) (u32, u32) {
    sum64 := u64(x) + u64(y) + u64(carry)
    sum := u32(sum64)
    carry_out := u32(sum64 >> 32)
    return sum, carry_out
}

// add_64 returns the sum with carry of x, y and carry: sum = x + y + carry.
// The carry input must be 0 or 1; otherwise the behavior is undefined.
// The carryOut output is guaranteed to be 0 or 1.
// This function's execution time does not depend on the inputs.
@[ignore_overflow]
pub fn add_64(x u64, y u64, carry u64) (u64, u64) {
    sum := x + y + carry
    // The sum will overflow if both top bits are set (x & y) or if one of them
    // is (x | y), and a carry from the lower place happened. If such a carry
    // happens, the top bit will be 1 + 0 + 1 = 0 (&^ sum).
    carry_out := ((x & y) | ((x | y) & ~sum)) >> 63
    return sum, carry_out
}

// --- Subtract with borrow ---
// Sub returns the difference of x, y and borrow: diff = x - y - borrow.
// The borrow input must be 0 or 1; otherwise the behavior is undefined.
// The borrowOut output is guaranteed to be 0 or 1.
// sub_32 returns the difference of x, y and borrow, diff = x - y - borrow.
// The borrow input must be 0 or 1; otherwise the behavior is undefined.
// The borrowOut output is guaranteed to be 0 or 1.
// This function's execution time does not depend on the inputs.
@[ignore_overflow]
pub fn sub_32(x u32, y u32, borrow u32) (u32, u32) {
    diff := x - y - borrow
    // The difference will underflow if the top bit of x is not set and the top
    // bit of y is set (^x & y) or if they are the same (^(x ^ y)) and a borrow
    // from the lower place happens. If that borrow happens, the result will be
    // 1 - 1 - 1 = 0 - 0 - 1 = 1 (& diff).
    borrow_out := ((~x & y) | (~(x ^ y) & diff)) >> 31
    return diff, borrow_out
}

// sub_64 returns the difference of x, y and borrow: diff = x - y - borrow.
// The borrow input must be 0 or 1; otherwise the behavior is undefined.
// The borrowOut output is guaranteed to be 0 or 1.
// This function's execution time does not depend on the inputs.
@[ignore_overflow]
pub fn sub_64(x u64, y u64, borrow u64) (u64, u64) {
    diff := x - y - borrow
    // See Sub32 for the bit logic.
    borrow_out := ((~x & y) | (~(x ^ y) & diff)) >> 63
    return diff, borrow_out
}

// --- Full-width multiply ---

@[markused]
pub const two32 = u64(0x100000000)
const mask32 = two32 - 1
const overflow_error = 'Overflow Error'
const divide_error = 'Divide Error'

// mul_32 returns the 64-bit product of x and y: (hi, lo) = x * y
// with the product bits' upper half returned in hi and the lower
// half returned in lo.
//
// This function's execution time does not depend on the inputs.
@[inline]
pub fn mul_32(x u32, y u32) (u32, u32) {
    return mul_32_default(x, y)
}

@[inline]
fn mul_32_default(x u32, y u32) (u32, u32) {
    tmp := u64(x) * u64(y)
    hi := u32(tmp >> 32)
    lo := u32(tmp)
    return hi, lo
}

// mul_64 returns the 128-bit product of x and y: (hi, lo) = x * y
// with the product bits' upper half returned in hi and the lower
// half returned in lo.
//
// This function's execution time does not depend on the inputs.
@[inline]
pub fn mul_64(x u64, y u64) (u64, u64) {
    return mul_64_default(x, y)
}

fn mul_64_default(x u64, y u64) (u64, u64) {
    x0 := x & mask32
    x1 := x >> 32
    y0 := y & mask32
    y1 := y >> 32
    w0 := x0 * y0
    t := x1 * y0 + (w0 >> 32)
    mut w1 := t & mask32
    w2 := t >> 32
    w1 += x0 * y1
    hi := x1 * y1 + w2 + (w1 >> 32)
    lo := x * y
    return hi, lo
}

// mul_add_32 returns the 64-bit result of x * y + z: (hi, lo) = x * y + z
// with the result bits' upper half returned in hi and the lower
// half returned in lo.
@[inline]
pub fn mul_add_32(x u32, y u32, z u32) (u32, u32) {
    return mul_add_32_default(x, y, z)
}

@[inline]
fn mul_add_32_default(x u32, y u32, z u32) (u32, u32) {
    tmp := u64(x) * u64(y) + u64(z)
    hi := u32(tmp >> 32)
    lo := u32(tmp)
    return hi, lo
}

// mul_add_64 returns the 128-bit result of x * y + z: (hi, lo) = x * y + z
// with the result bits' upper half returned in hi and the lower
// half returned in lo.
@[inline]
pub fn mul_add_64(x u64, y u64, z u64) (u64, u64) {
    return mul_add_64_default(x, y, z)
}

@[inline]
fn mul_add_64_default(x u64, y u64, z u64) (u64, u64) {
    h, l := mul_64(x, y)
    lo := l + z
    hi := h + u64(lo < l)
    return hi, lo
}

// --- Full-width divide ---
// div_32 returns the quotient and remainder of (hi, lo) divided by y:
// quo = (hi, lo)/y, rem = (hi, lo)%y with the dividend bits' upper
// half in parameter hi and the lower half in parameter lo.
// div_32 panics for y == 0 (division by zero) or y <= hi (quotient overflow).
@[inline]
pub fn div_32(hi u32, lo u32, y u32) (u32, u32) {
    return div_32_default(hi, lo, y)
}

fn div_32_default(hi u32, lo u32, y u32) (u32, u32) {
    if y != 0 && y <= hi {
        panic(overflow_error)
    }
    z := (u64(hi) << 32) | u64(lo)
    quo := u32(z / u64(y))
    rem := u32(z % u64(y))
    return quo, rem
}

// div_64 returns the quotient and remainder of (hi, lo) divided by y:
// quo = (hi, lo)/y, rem = (hi, lo)%y with the dividend bits' upper
// half in parameter hi and the lower half in parameter lo.
// div_64 panics for y == 0 (division by zero) or y <= hi (quotient overflow).
@[inline]
pub fn div_64(hi u64, lo u64, y1 u64) (u64, u64) {
    return div_64_default(hi, lo, y1)
}

fn div_64_default(hi u64, lo u64, y1 u64) (u64, u64) {
    mut y := y1
    if y == 0 {
        panic(overflow_error)
    }
    if y <= hi {
        panic(overflow_error)
    }
    s := u32(leading_zeros_64(y))
    y <<= s
    yn1 := y >> 32
    yn0 := y & mask32
    ss1 := (hi << s)
    xxx := 64 - s
    mut ss2 := lo >> xxx
    if xxx == 64 {
        // in Go, shifting right a u64 number, 64 times produces 0 *always*.
        // See https://go.dev/ref/spec
        // > The shift operators implement arithmetic shifts if the left operand
        // > is a signed integer and logical shifts if it is an unsigned integer.
        // > There is no upper limit on the shift count.
        // > Shifts behave as if the left operand is shifted n times by 1 for a shift count of n.
        // > As a result, x << 1 is the same as x*2 and x >> 1 is the same as x/2
        // > but truncated towards negative infinity.
        //
        // In V, that is currently left to whatever C is doing, which is apparently a NOP.
        // This function was a direct port of https://cs.opensource.google/go/go/+/refs/tags/go1.17.6:src/math/bits/bits.go;l=512,
        // so we have to use the Go behaviour.
        // TODO: reconsider whether we need to adopt it for our shift ops, or just use function wrappers that do it.
        ss2 = 0
    }
    un32 := ss1 | ss2
    un10 := lo << s
    un1 := un10 >> 32
    un0 := un10 & mask32
    mut q1 := un32 / yn1
    mut rhat := un32 - (q1 * yn1)
    for q1 >= two32 || (q1 * yn0) > ((two32 * rhat) + un1) {
        q1--
        rhat += yn1
        if rhat >= two32 {
            break
        }
    }
    un21 := (un32 * two32) + (un1 - (q1 * y))
    mut q0 := un21 / yn1
    rhat = un21 - q0 * yn1
    for q0 >= two32 || (q0 * yn0) > ((two32 * rhat) + un0) {
        q0--
        rhat += yn1
        if rhat >= two32 {
            break
        }
    }
    qq := ((q1 * two32) + q0)
    rr := ((un21 * two32) + un0 - (q0 * y)) >> s
    return qq, rr
}

// rem_32 returns the remainder of (hi, lo) divided by y. Rem32 panics
// for y == 0 (division by zero) but, unlike Div32, it doesn't panic
// on a quotient overflow.
pub fn rem_32(hi u32, lo u32, y u32) u32 {
    return u32(((u64(hi) << 32) | u64(lo)) % u64(y))
}

// rem_64 returns the remainder of (hi, lo) divided by y. Rem64 panics
// for y == 0 (division by zero) but, unlike div_64, it doesn't panic
// on a quotient overflow.
pub fn rem_64(hi u64, lo u64, y u64) u64 {
    // We scale down hi so that hi < y, then use div_64 to compute the
    // rem with the guarantee that it won't panic on quotient overflow.
    // Given that
    // hi ≡ hi%y    (mod y)
    // we have
    // hi<<64 + lo ≡ (hi%y)<<64 + lo    (mod y)
    _, rem := div_64(hi % y, lo, y)
    return rem
}

// normalize returns a normal number y and exponent exp
// satisfying x == y × 2**exp. It assumes x is finite and non-zero.
pub fn normalize(x f64) (f64, int) {
    smallest_normal := 2.2250738585072014e-308 // 2**-1022
    if (if x > 0.0 {
        x
    } else {
        -x
    }) < smallest_normal {
        return x * (u64(1) << u64(52)), -52
    }
    return x, 0
}