comment non-obvious de bruijn sequence code in int parser

diff --git a/src/internal/intparse.c b/src/internal/intparse.c
index fd403b589320633744d58fff0a17c3c7b366d9d4..90aa8339f1a35e48a0dfe55218aafa58a99746ad 100644 (file)
--- a/src/internal/intparse.c
+++ b/src/internal/intparse.c
@@ -70,6 +70,8 @@ int __intparse(struct intparse *v, const void *buf, size_t n)
                        for (; n && *s-'0'<10U && v->small<=SLIM; n--, s++)
                                v->small = v->small * 10 + (*s-'0');
                } else if ((b&-b) == b) {
+                       /* Compute bitshift for power-of-two bases
+                        * using a De Bruijn B(2,3) sequence. */
                        int bs = "\0\1\2\4\7\3\6\5"[(0x17*b)>>5&7];
                        for (; n && (d=digits[*s])<b && v->small<=SLIM; n--, s++)
                                v->small = (v->small<<bs) + d;