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// 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

// FormatUint returns the string representation of i in the given base.
func FormatUint(i uint64, base int) string {
        _, s := formatBits(nil, i, base, false, false)
        return s
}

// FormatInt returns the string representation of i in the given base.
func FormatInt(i int64, base int) string {
        _, s := formatBits(nil, uint64(i), base, i < 0, false)
        return s
}

// Itoa is shorthand for FormatInt(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 {
        dst, _ = formatBits(dst, uint64(i), base, i < 0, true)
        return dst
}

// 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 {
        dst, _ = formatBits(dst, i, base, false, true)
        return dst
}

const (
        digits   = "0123456789abcdefghijklmnopqrstuvwxyz"
        digits01 = "0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789"
        digits10 = "0000000000111111111122222222223333333333444444444455555555556666666666777777777788888888889999999999"
)

var shifts = [len(digits) + 1]uint{
        1 << 1: 1,
        1 << 2: 2,
        1 << 3: 3,
        1 << 4: 4,
        1 << 5: 5,
}

// 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.
//
func formatBits(dst []byte, u uint64, base int, neg, append_ bool) (d []byte, s string) {
        if base < 2 || 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
        if base == 10 {
                // common case: use constants for / and % because
                // the compiler can optimize it into a multiply+shift,
                // and unroll loop
                for u >= 100 {
                        i -= 2
                        q := u / 100
                        j := uintptr(u - q*100)
                        a[i+1] = digits01[j]
                        a[i+0] = digits10[j]
                        u = q
                }
                if u >= 10 {
                        i--
                        q := u / 10
                        a[i] = digits[uintptr(u-q*10)]
                        u = q
                }

        } else if s := shifts[base]; s > 0 {
                // base is power of 2: use shifts and masks instead of / and %
                b := uint64(base)
                m := uintptr(b) - 1 // == 1<<s - 1
                for u >= b {
                        i--
                        a[i] = digits[uintptr(u)&m]
                        u >>= s
                }

        } else {
                // general case
                b := uint64(base)
                for u >= b {
                        i--
                        a[i] = digits[uintptr(u%b)]
                        u /= b
                }
        }

        // u < base
        i--
        a[i] = digits[uintptr(u)]

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

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