1 files changed, 91 insertions, 0 deletions
diff --git a/pkg/gitdiff/base85.go b/pkg/gitdiff/base85.go
new file mode 100644
index 0000000..86db117
--- /dev/null
+++ b/pkg/gitdiff/base85.go
@@ -0,0 +1,91 @@
+package gitdiff
+
+import (
+	"fmt"
+)
+
+var (
+	b85Table map[byte]byte
+	b85Alpha = []byte(
+		"0123456789" + "ABCDEFGHIJKLMNOPQRSTUVWXYZ" + "abcdefghijklmnopqrstuvwxyz" + "!#$%&()*+-;<=>?@^_`{|}~",
+	)
+)
+
+func init() {
+	b85Table = make(map[byte]byte)
+	for i, c := range b85Alpha {
+		b85Table[c] = byte(i)
+	}
+}
+
+// base85Decode decodes Base85-encoded data from src into dst. It uses the
+// alphabet defined by base85.c in the Git source tree. src must contain at
+// least len(dst) bytes of encoded data.
+func base85Decode(dst, src []byte) error {
+	var v uint32
+	var n, ndst int
+	for i, b := range src {
+		if b, ok := b85Table[b]; ok {
+			v = 85*v + uint32(b)
+			n++
+		} else {
+			return fmt.Errorf("invalid base85 byte at index %d: 0x%X", i, src[i])
+		}
+		if n == 5 {
+			rem := len(dst) - ndst
+			for j := 0; j < 4 && j < rem; j++ {
+				dst[ndst] = byte(v >> 24)
+				ndst++
+				v <<= 8
+			}
+			v = 0
+			n = 0
+		}
+	}
+	if n > 0 {
+		return fmt.Errorf("base85 data terminated by underpadded sequence")
+	}
+	if ndst < len(dst) {
+		return fmt.Errorf("base85 data underrun: %d < %d", ndst, len(dst))
+	}
+	return nil
+}
+
+// base85Encode encodes src in Base85, writing the result to dst. It uses the
+// alphabet defined by base85.c in the Git source tree.
+func base85Encode(dst, src []byte) {
+	var di, si int
+
+	encode := func(v uint32) {
+		dst[di+0] = b85Alpha[(v/(85*85*85*85))%85]
+		dst[di+1] = b85Alpha[(v/(85*85*85))%85]
+		dst[di+2] = b85Alpha[(v/(85*85))%85]
+		dst[di+3] = b85Alpha[(v/85)%85]
+		dst[di+4] = b85Alpha[v%85]
+	}
+
+	n := (len(src) / 4) * 4
+	for si < n {
+		encode(uint32(src[si+0])<<24 | uint32(src[si+1])<<16 | uint32(src[si+2])<<8 | uint32(src[si+3]))
+		si += 4
+		di += 5
+	}
+
+	var v uint32
+	switch len(src) - si {
+	case 3:
+		v |= uint32(src[si+2]) << 8
+		fallthrough
+	case 2:
+		v |= uint32(src[si+1]) << 16
+		fallthrough
+	case 1:
+		v |= uint32(src[si+0]) << 24
+		encode(v)
+	}
+}
+
+// base85Len returns the length of n bytes of Base85 encoded data.
+func base85Len(n int) int {
+	return (n + 3) / 4 * 5
+}