package plumbing

// Adapted from https://github.com/kr/binarydist
// Original license:
//
// Copyright 2012 Keith Rarick
//
// Permission is hereby granted, free of charge, to any person
// obtaining a copy of this software and associated documentation
// files (the "Software"), to deal in the Software without
// restriction, including without limitation the rights to use,
// copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the
// Software is furnished to do so, subject to the following
// conditions:
//
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
// OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
// HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
// WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
// OTHER DEALINGS IN THE SOFTWARE.

import (
	"bytes"
)

func swap(a []int, i, j int) { a[i], a[j] = a[j], a[i] }

func split(I, V []int, start, length, h int) {
	var i, j, k, x, jj, kk int

	if length < 16 {
		for k = start; k < start+length; k += j {
			j = 1
			x = V[I[k]+h]
			for i = 1; k+i < start+length; i++ {
				if V[I[k+i]+h] < x {
					x = V[I[k+i]+h]
					j = 0
				}
				if V[I[k+i]+h] == x {
					swap(I, k+i, k+j)
					j++
				}
			}
			for i = 0; i < j; i++ {
				V[I[k+i]] = k + j - 1
			}
			if j == 1 {
				I[k] = -1
			}
		}
		return
	}

	x = V[I[start+length/2]+h]
	jj = 0
	kk = 0
	for i = start; i < start+length; i++ {
		if V[I[i]+h] < x {
			jj++
		}
		if V[I[i]+h] == x {
			kk++
		}
	}
	jj += start
	kk += jj

	i = start
	j = 0
	k = 0
	for i < jj {
		if V[I[i]+h] < x {
			i++
		} else if V[I[i]+h] == x {
			swap(I, i, jj+j)
			j++
		} else {
			swap(I, i, kk+k)
			k++
		}
	}

	for jj+j < kk {
		if V[I[jj+j]+h] == x {
			j++
		} else {
			swap(I, jj+j, kk+k)
			k++
		}
	}

	if jj > start {
		split(I, V, start, jj-start, h)
	}

	for i = 0; i < kk-jj; i++ {
		V[I[jj+i]] = kk - 1
	}
	if jj == kk-1 {
		I[jj] = -1
	}

	if start+length > kk {
		split(I, V, kk, start+length-kk, h)
	}
}

func qsufsort(obuf []byte) []int {
	var buckets [256]int
	var i, h int
	I := make([]int, len(obuf)+1)
	V := make([]int, len(obuf)+1)

	for _, c := range obuf {
		buckets[c]++
	}
	for i = 1; i < 256; i++ {
		buckets[i] += buckets[i-1]
	}
	copy(buckets[1:], buckets[:])
	buckets[0] = 0

	for i, c := range obuf {
		buckets[c]++
		I[buckets[c]] = i
	}

	I[0] = len(obuf)
	for i, c := range obuf {
		V[i] = buckets[c]
	}

	V[len(obuf)] = 0
	for i = 1; i < 256; i++ {
		if buckets[i] == buckets[i-1]+1 {
			I[buckets[i]] = -1
		}
	}
	I[0] = -1

	for h = 1; I[0] != -(len(obuf) + 1); h += h {
		var n int
		for i = 0; i < len(obuf)+1; {
			if I[i] < 0 {
				n -= I[i]
				i -= I[i]
			} else {
				if n != 0 {
					I[i-n] = -n
				}
				n = V[I[i]] + 1 - i
				split(I, V, i, n, h)
				i += n
				n = 0
			}
		}
		if n != 0 {
			I[i-n] = -n
		}
	}

	for i = 0; i < len(obuf)+1; i++ {
		I[V[i]] = i
	}
	return I
}

func matchlen(a, b []byte) (i int) {
	for i < len(a) && i < len(b) && a[i] == b[i] {
		i++
	}
	return i
}

func search(I []int, obuf, nbuf []byte, st, en int) (pos, n int) {
	if en-st < 2 {
		x := matchlen(obuf[I[st]:], nbuf)
		y := matchlen(obuf[I[en]:], nbuf)

		if x > y {
			return I[st], x
		}
		return I[en], y
	}

	x := st + (en-st)/2
	if bytes.Compare(obuf[I[x]:], nbuf) < 0 {
		return search(I, obuf, nbuf, x, en)
	}
	return search(I, obuf, nbuf, st, x)
}

// DiffBytes calculates the approximated number of different bytes between two binary buffers.
// We are not interested in the diff script itself. Instead, we track the sizes of `db` and `eb`
// from the original implementation.
func DiffBytes(obuf, nbuf []byte) int {
	if len(nbuf) < len(obuf) {
		obuf, nbuf = nbuf, obuf
	}
	var lenf int
	I := qsufsort(obuf)
	var dblen, eblen int

	// Compute the differences, writing ctrl as we go
	var scan, pos, length int
	var lastscan, lastpos, lastoffset int
	for scan < len(nbuf) {
		var oldscore int
		scan += length
		for scsc := scan; scan < len(nbuf); scan++ {
			pos, length = search(I, obuf, nbuf[scan:], 0, len(obuf))

			for ; scsc < scan+length; scsc++ {
				if scsc+lastoffset < len(obuf) &&
					obuf[scsc+lastoffset] == nbuf[scsc] {
					oldscore++
				}
			}

			if (length == oldscore && length != 0) || length > oldscore+8 {
				break
			}

			if scan+lastoffset < len(obuf) && obuf[scan+lastoffset] == nbuf[scan] {
				oldscore--
			}
		}

		if length != oldscore || scan == len(nbuf) {
			var s, Sf int
			lenf = 0
			for i := 0; lastscan+i < scan && lastpos+i < len(obuf); {
				if obuf[lastpos+i] == nbuf[lastscan+i] {
					s++
				}
				i++
				if s*2-i > Sf*2-lenf {
					Sf = s
					lenf = i
				}
			}

			lenb := 0
			if scan < len(nbuf) {
				var s, Sb int
				for i := 1; (scan >= lastscan+i) && (pos >= i); i++ {
					if obuf[pos-i] == nbuf[scan-i] {
						s++
					}
					if s*2-i > Sb*2-lenb {
						Sb = s
						lenb = i
					}
				}
			}

			if lastscan+lenf > scan-lenb {
				overlap := (lastscan + lenf) - (scan - lenb)
				s := 0
				Ss := 0
				lens := 0
				for i := 0; i < overlap; i++ {
					if nbuf[lastscan+lenf-overlap+i] == obuf[lastpos+lenf-overlap+i] {
						s++
					}
					if nbuf[scan-lenb+i] == obuf[pos-lenb+i] {
						s--
					}
					if s > Ss {
						Ss = s
						lens = i + 1
					}
				}

				lenf += lens - overlap
				lenb -= lens
			}

			var nonzero int
			for i := 0; i < lenf; i++ {
				if nbuf[lastscan+i]-obuf[lastpos+i] != 0 {
					nonzero++
				}
			}

			dblen += nonzero
			eblen += (scan - lenb) - (lastscan + lenf)
			lastscan = scan - lenb
			lastpos = pos - lenb
			lastoffset = pos - scan
		}
	}
	return dblen + eblen
}