src/internal/strconv/itoa.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package strconv

import "math/bits"

// FormatUint returns the string representation of i in the given base,
// for 2 <= base <= 36. The result uses the lower-case letters 'a' to 'z'
// for digit values >= 10.
func FormatUint(i uint64, base int) string {
	if base == 10 {
		if i < nSmalls {
			return small(int(i))
		}
		var a [24]byte
		j := formatBase10(a[:], i)
		return string(a[j:])
	}
	_, s := formatBits(nil, i, base, false, false)
	return s
}

// FormatInt returns the string representation of i in the given base,
// for 2 <= base <= 36. The result uses the lower-case letters 'a' to 'z'
// for digit values >= 10.
func FormatInt(i int64, base int) string {
	if base == 10 {
		if 0 <= i && i < nSmalls {
			return small(int(i))
		}
		var a [24]byte
		u := uint64(i)
		if i < 0 {
			u = -u
		}
		j := formatBase10(a[:], u)
		if i < 0 {
			j--
			a[j] = '-'
		}
		return string(a[j:])
	}
	_, s := formatBits(nil, uint64(i), base, i < 0, false)
	return s
}

// Itoa is equivalent to [FormatInt](int64(i), 10).
func Itoa(i int) string {
	return FormatInt(int64(i), 10)
}

// AppendInt appends the string form of the integer i,
// as generated by [FormatInt], to dst and returns the extended buffer.
func AppendInt(dst []byte, i int64, base int) []byte {
	u := uint64(i)
	if i < 0 {
		dst = append(dst, '-')
		u = -u
	}
	return AppendUint(dst, u, base)
}

// AppendUint appends the string form of the unsigned integer i,
// as generated by [FormatUint], to dst and returns the extended buffer.
func AppendUint(dst []byte, i uint64, base int) []byte {
	if base == 10 {
		if i < nSmalls {
			return append(dst, small(int(i))...)
		}
		var a [24]byte
		j := formatBase10(a[:], i)
		return append(dst, a[j:]...)
	}
	dst, _ = formatBits(dst, i, base, false, true)
	return dst
}

const digits = "0123456789abcdefghijklmnopqrstuvwxyz"

// formatBits computes the string representation of u in the given base.
// If neg is set, u is treated as negative int64 value. If append_ is
// set, the string is appended to dst and the resulting byte slice is
// returned as the first result value; otherwise the string is returned
// as the second result value.
// The caller is expected to have handled base 10 separately for speed.
func formatBits(dst []byte, u uint64, base int, neg, append_ bool) (d []byte, s string) {
	if base < 2 || base == 10 || base > len(digits) {
		panic("strconv: illegal AppendInt/FormatInt base")
	}
	// 2 <= base && base <= len(digits)

	var a [64 + 1]byte // +1 for sign of 64bit value in base 2
	i := len(a)
	if neg {
		u = -u
	}

	// convert bits
	// We use uint values where we can because those will
	// fit into a single register even on a 32bit machine.
	if isPowerOfTwo(base) {
		// Use shifts and masks instead of / and %.
		shift := uint(bits.TrailingZeros(uint(base)))
		b := uint64(base)
		m := uint(base) - 1 // == 1<<shift - 1
		for u >= b {
			i--
			a[i] = digits[uint(u)&m]
			u >>= shift
		}
		// u < base
		i--
		a[i] = digits[uint(u)]
	} else {
		// general case
		b := uint64(base)
		for u >= b {
			i--
			// Avoid using r = a%b in addition to q = a/b
			// since 64bit division and modulo operations
			// are calculated by runtime functions on 32bit machines.
			q := u / b
			a[i] = digits[uint(u-q*b)]
			u = q
		}
		// u < base
		i--
		a[i] = digits[uint(u)]
	}

	// add sign, if any
	if neg {
		i--
		a[i] = '-'
	}

	if append_ {
		d = append(dst, a[i:]...)
		return
	}
	s = string(a[i:])
	return
}

func isPowerOfTwo(x int) bool {
	return x&(x-1) == 0
}

const nSmalls = 100

// smalls is the formatting of 00..99 concatenated.
// It is then padded out with 56 x's to 256 bytes,
// so that smalls[x&0xFF] has no bounds check.
const smalls = "00010203040506070809" +
	"10111213141516171819" +
	"20212223242526272829" +
	"30313233343536373839" +
	"40414243444546474849" +
	"50515253545556575859" +
	"60616263646566676869" +
	"70717273747576777879" +
	"80818283848586878889" +
	"90919293949596979899"

const host64bit = ^uint(0)>>32 != 0

// small returns the string for an i with 0 <= i < nSmalls.
func small(i int) string {
	if i < 10 {
		return digits[i : i+1]
	}
	return smalls[i*2 : i*2+2]
}

// RuntimeFormatBase10 formats u into the tail of a
// and returns the offset to the first byte written to a.
// It is only for use by package runtime.
// Other packages should use AppendUint.
func RuntimeFormatBase10(a []byte, u uint64) int {
	return formatBase10(a, u)
}

// formatBase10 formats the decimal representation of u into the tail of a
// and returns the offset of the first byte written to a. That is, after
//
//	i := formatBase10(a, u)
//
// the decimal representation is in a[i:].
func formatBase10(a []byte, u uint64) int {
	// Split into 9-digit chunks that fit in uint32s
	// and convert each chunk using uint32 math instead of uint64 math.
	// The obvious way to write the outer loop is "for u >= 1e9", but most numbers are small,
	// so the setup for the comparison u >= 1e9 is usually pure overhead.
	// Instead, we approximate it by u>>29 != 0, which is usually faster and good enough.
	i := len(a)
	for (host64bit && u>>29 != 0) || (!host64bit && uint32(u)>>29|uint32(u>>32) != 0) {
		var lo uint32
		u, lo = u/1e9, uint32(u%1e9)

		// Convert 9 digits.
		for range 4 {
			var dd uint32
			lo, dd = lo/100, (lo%100)*2
			i -= 2
			a[i+0], a[i+1] = smalls[dd+0], smalls[dd+1]
		}
		i--
		a[i] = smalls[lo*2+1]

		// If we'd been using u >= 1e9 then we would be guaranteed that u/1e9 > 0,
		// but since we used u>>29 != 0, u/1e9 might be 0, so we might be done.
		// (If u is now 0, then at the start we had 2²⁹ ≤ u < 10⁹, so it was still correct
		// to write 9 digits; we have not accidentally written any leading zeros.)
		if u == 0 {
			return i
		}
	}

	// Convert final chunk, at most 8 digits.
	lo := uint32(u)
	for lo >= 100 {
		var dd uint32
		lo, dd = lo/100, (lo%100)*2
		i -= 2
		a[i+0], a[i+1] = smalls[dd+0], smalls[dd+1]
	}
	i--
	dd := lo * 2
	a[i] = smalls[dd+1]
	if lo >= 10 {
		i--
		a[i] = smalls[dd+0]
	}
	return i
}