1 files changed, 241 insertions, 0 deletions
diff --git a/Godeps/_workspace/src/github.com/mattermost/rsc/gf256/gf256.go b/Godeps/_workspace/src/github.com/mattermost/rsc/gf256/gf256.go
new file mode 100644
index 000000000..34cc975a8
--- /dev/null
+++ b/Godeps/_workspace/src/github.com/mattermost/rsc/gf256/gf256.go
@@ -0,0 +1,241 @@
+// Copyright 2010 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 gf256 implements arithmetic over the Galois Field GF(256).
+package gf256
+
+import "strconv"
+
+// A Field represents an instance of GF(256) defined by a specific polynomial.
+type Field struct {
+	log [256]byte // log[0] is unused
+	exp [510]byte
+}
+
+// NewField returns a new field corresponding to the polynomial poly
+// and generator α.  The Reed-Solomon encoding in QR codes uses
+// polynomial 0x11d with generator 2.
+//
+// The choice of generator α only affects the Exp and Log operations.
+func NewField(poly, α int) *Field {
+	if poly < 0x100 || poly >= 0x200 || reducible(poly) {
+		panic("gf256: invalid polynomial: " + strconv.Itoa(poly))
+	}
+
+	var f Field
+	x := 1
+	for i := 0; i < 255; i++ {
+		if x == 1 && i != 0 {
+			panic("gf256: invalid generator " + strconv.Itoa(α) +
+				" for polynomial " + strconv.Itoa(poly))
+		}
+		f.exp[i] = byte(x)
+		f.exp[i+255] = byte(x)
+		f.log[x] = byte(i)
+		x = mul(x, α, poly)
+	}
+	f.log[0] = 255
+	for i := 0; i < 255; i++ {
+		if f.log[f.exp[i]] != byte(i) {
+			panic("bad log")
+		}
+		if f.log[f.exp[i+255]] != byte(i) {
+			panic("bad log")
+		}
+	}
+	for i := 1; i < 256; i++ {
+		if f.exp[f.log[i]] != byte(i) {
+			panic("bad log")
+		}
+	}
+
+	return &f
+}
+
+// nbit returns the number of significant in p.
+func nbit(p int) uint {
+	n := uint(0)
+	for ; p > 0; p >>= 1 {
+		n++
+	}
+	return n
+}
+
+// polyDiv divides the polynomial p by q and returns the remainder.
+func polyDiv(p, q int) int {
+	np := nbit(p)
+	nq := nbit(q)
+	for ; np >= nq; np-- {
+		if p&(1<<(np-1)) != 0 {
+			p ^= q << (np - nq)
+		}
+	}
+	return p
+}
+
+// mul returns the product x*y mod poly, a GF(256) multiplication.
+func mul(x, y, poly int) int {
+	z := 0
+	for x > 0 {
+		if x&1 != 0 {
+			z ^= y
+		}
+		x >>= 1
+		y <<= 1
+		if y&0x100 != 0 {
+			y ^= poly
+		}
+	}
+	return z
+}
+
+// reducible reports whether p is reducible.
+func reducible(p int) bool {
+	// Multiplying n-bit * n-bit produces (2n-1)-bit,
+	// so if p is reducible, one of its factors must be
+	// of np/2+1 bits or fewer.
+	np := nbit(p)
+	for q := 2; q < 1<<(np/2+1); q++ {
+		if polyDiv(p, q) == 0 {
+			return true
+		}
+	}
+	return false
+}
+
+// Add returns the sum of x and y in the field.
+func (f *Field) Add(x, y byte) byte {
+	return x ^ y
+}
+
+// Exp returns the the base-α exponential of e in the field.
+// If e < 0, Exp returns 0.
+func (f *Field) Exp(e int) byte {
+	if e < 0 {
+		return 0
+	}
+	return f.exp[e%255]
+}
+
+// Log returns the base-α logarithm of x in the field.
+// If x == 0, Log returns -1.
+func (f *Field) Log(x byte) int {
+	if x == 0 {
+		return -1
+	}
+	return int(f.log[x])
+}
+
+// Inv returns the multiplicative inverse of x in the field.
+// If x == 0, Inv returns 0.
+func (f *Field) Inv(x byte) byte {
+	if x == 0 {
+		return 0
+	}
+	return f.exp[255-f.log[x]]
+}
+
+// Mul returns the product of x and y in the field.
+func (f *Field) Mul(x, y byte) byte {
+	if x == 0 || y == 0 {
+		return 0
+	}
+	return f.exp[int(f.log[x])+int(f.log[y])]
+}
+
+// An RSEncoder implements Reed-Solomon encoding
+// over a given field using a given number of error correction bytes.
+type RSEncoder struct {
+	f    *Field
+	c    int
+	gen  []byte
+	lgen []byte
+	p    []byte
+}
+
+func (f *Field) gen(e int) (gen, lgen []byte) {
+	// p = 1
+	p := make([]byte, e+1)
+	p[e] = 1
+
+	for i := 0; i < e; i++ {
+		// p *= (x + Exp(i))
+		// p[j] = p[j]*Exp(i) + p[j+1].
+		c := f.Exp(i)
+		for j := 0; j < e; j++ {
+			p[j] = f.Mul(p[j], c) ^ p[j+1]
+		}
+		p[e] = f.Mul(p[e], c)
+	}
+
+	// lp = log p.
+	lp := make([]byte, e+1)
+	for i, c := range p {
+		if c == 0 {
+			lp[i] = 255
+		} else {
+			lp[i] = byte(f.Log(c))
+		}
+	}
+
+	return p, lp
+}
+
+// NewRSEncoder returns a new Reed-Solomon encoder
+// over the given field and number of error correction bytes.
+func NewRSEncoder(f *Field, c int) *RSEncoder {
+	gen, lgen := f.gen(c)
+	return &RSEncoder{f: f, c: c, gen: gen, lgen: lgen}
+}
+
+// ECC writes to check the error correcting code bytes
+// for data using the given Reed-Solomon parameters.
+func (rs *RSEncoder) ECC(data []byte, check []byte) {
+	if len(check) < rs.c {
+		panic("gf256: invalid check byte length")
+	}
+	if rs.c == 0 {
+		return
+	}
+
+	// The check bytes are the remainder after dividing
+	// data padded with c zeros by the generator polynomial.  
+
+	// p = data padded with c zeros.
+	var p []byte
+	n := len(data) + rs.c
+	if len(rs.p) >= n {
+		p = rs.p
+	} else {
+		p = make([]byte, n)
+	}
+	copy(p, data)
+	for i := len(data); i < len(p); i++ {
+		p[i] = 0
+	}
+
+	// Divide p by gen, leaving the remainder in p[len(data):].
+	// p[0] is the most significant term in p, and
+	// gen[0] is the most significant term in the generator,
+	// which is always 1.
+	// To avoid repeated work, we store various values as
+	// lv, not v, where lv = log[v].
+	f := rs.f
+	lgen := rs.lgen[1:]
+	for i := 0; i < len(data); i++ {
+		c := p[i]
+		if c == 0 {
+			continue
+		}
+		q := p[i+1:]
+		exp := f.exp[f.log[c]:]
+		for j, lg := range lgen {
+			if lg != 255 { // lgen uses 255 for log 0
+				q[j] ^= exp[lg]
+			}
+		}
+	}
+	copy(check, p[len(data):])
+	rs.p = p
+}