21696: Comment optimized PortableDataHash func.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@curii.com>

diff --git a/sdk/go/arvados/collection.go b/sdk/go/arvados/collection.go
index b157c4c8a8dc8a2dd5963d2bd914d7bbd8eb04a5..1e9616c428064c6e5bdbf30c7a3b4d46d79956bf 100644 (file)
--- a/sdk/go/arvados/collection.go
+++ b/sdk/go/arvados/collection.go
@@ -107,27 +107,49 @@ type CollectionList struct {
 // PortableDataHash computes the portable data hash of the given
 // manifest.
 func PortableDataHash(mt string) string {
+       // To calculate the PDH, we write the manifest to an md5 hash
+       // func, except we skip the "extra" part of block tokens that
+       // look like "abcdef0123456789abcdef0123456789+12345+extra".
+       //
+       // This code is simplified by the facts that (A) all block
+       // tokens -- even the first and last in a stream -- are
+       // preceded and followed by a space character; and (B) all
+       // non-block tokens either start with '.'  or contain ':'.
+       //
+       // A regexp-based approach (like the one this replaced) would
+       // be more readable, but very slow.
        h := md5.New()
        size := 0
        todo := []byte(mt)
        for len(todo) > 0 {
+               // sp is the end of the current token (note that if
+               // the current token is the last file token in a
+               // stream, we'll also include the \n and the dirname
+               // token on the next line, which is perfectly fine for
+               // our purposes).
                sp := bytes.IndexByte(todo, ' ')
                if sp < 0 {
+                       // Last token of the manifest, which is never
+                       // a block token.
                        n, _ := h.Write(todo)
                        size += n
                        break
                }
-               if sp >= 34 && todo[32] == '+' && bytes.IndexByte(todo[:32], ':') == -1 {
+               if sp >= 34 && todo[32] == '+' && bytes.IndexByte(todo[:32], ':') == -1 && todo[0] != '.' {
+                       // todo[:sp] is a block token.
                        sizeend := bytes.IndexByte(todo[33:sp], '+')
                        if sizeend < 0 {
+                               // "hash+size"
                                sizeend = sp
                        } else {
+                               // "hash+size+extra"
                                sizeend += 33
                        }
                        n, _ := h.Write(todo[:sizeend])
                        h.Write([]byte{' '})
                        size += n + 1
                } else {
+                       // todo[:sp] is not a block token.
                        n, _ := h.Write(todo[:sp+1])
                        size += n
                }