From abc610ff70dca2534fb17a2ca835707340d96cf1 Mon Sep 17 00:00:00 2001
From: tompng <tomoyapenguin@gmail.com>
Date: Tue, 10 Nov 2026 02:50:29 +0900
Subject: [PATCH] Select the special string candidate by its first character

The fast path added in #565 switches on the first character only to
skip the table, so the first characters are listed separately from the
table entries. Pick the candidate string in that switch instead of
looping over the table, so at most one strncmp is done.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 ext/bigdecimal/bigdecimal.c | 77 +++++++++++++++++--------------------
 1 file changed, 35 insertions(+), 42 deletions(-)

diff --git a/ext/bigdecimal/bigdecimal.c b/ext/bigdecimal/bigdecimal.c
index a4031e9e..2c28e13a 100644
--- a/ext/bigdecimal/bigdecimal.c
+++ b/ext/bigdecimal/bigdecimal.c
@@ -4142,57 +4142,50 @@ AddExponent(Real *a, SIGNED_VALUE n)
 NULLABLE_BDVALUE
 bigdecimal_parse_special_string(const char *str)
 {
-    static const struct {
-        const char *str;
-        size_t len;
-        int sign;
-    } table[] = {
-        { SZ_INF,  sizeof(SZ_INF)  - 1, VP_SIGN_POSITIVE_INFINITE },
-        { SZ_PINF, sizeof(SZ_PINF) - 1, VP_SIGN_POSITIVE_INFINITE },
-        { SZ_NINF, sizeof(SZ_NINF) - 1, VP_SIGN_NEGATIVE_INFINITE },
-        { SZ_NaN,  sizeof(SZ_NaN)  - 1, VP_SIGN_NaN               }
-    };
-    static const size_t table_length = sizeof(table) / sizeof(table[0]);
-    size_t i;
+    const char *word;
+    size_t len;
+    int sign;
 
-    /* Fast path: every entry of the table starts with one of these */
     switch (*str) {
-      case 'I': case 'N': case '+': case '-':
+      case 'I':
+        word = SZ_INF;
+        len = sizeof(SZ_INF) - 1;
+        sign = VP_SIGN_POSITIVE_INFINITE;
+        break;
+      case '+':
+        word = SZ_PINF;
+        len = sizeof(SZ_PINF) - 1;
+        sign = VP_SIGN_POSITIVE_INFINITE;
+        break;
+      case '-':
+        word = SZ_NINF;
+        len = sizeof(SZ_NINF) - 1;
+        sign = VP_SIGN_NEGATIVE_INFINITE;
+        break;
+      case 'N':
+        word = SZ_NaN;
+        len = sizeof(SZ_NaN) - 1;
+        sign = VP_SIGN_NaN;
         break;
       default:
         return (NULLABLE_BDVALUE) { Qnil, NULL };
     }
 
-    for (i = 0; i < table_length; ++i) {
-        const char *p;
-        if (strncmp(str, table[i].str, table[i].len) != 0) {
-            continue;
-        }
+    if (strncmp(str, word, len) != 0) return (NULLABLE_BDVALUE) { Qnil, NULL };
 
-        p = str + table[i].len;
-        while (*p && ISSPACE(*p)) ++p;
-        if (*p == '\0') {
-            VALUE obj = BigDecimal_allocate(1);
-            Real *vp = VpPtr(obj);
-            switch (table[i].sign) {
-              default:
-                UNREACHABLE;
-                return (NULLABLE_BDVALUE) { Qnil, NULL };
-              case VP_SIGN_POSITIVE_INFINITE:
-                VpSetPosInf(vp);
-                break;
-              case VP_SIGN_NEGATIVE_INFINITE:
-                VpSetNegInf(vp);
-                break;
-              case VP_SIGN_NaN:
-                VpSetNaN(vp);
-                break;
-            }
-            return (NULLABLE_BDVALUE) { obj, vp };
-        }
-    }
+    const char *p = str + len;
+    while (ISSPACE(*p)) ++p;
+    if (*p != '\0') return (NULLABLE_BDVALUE) { Qnil, NULL };
 
-    return (NULLABLE_BDVALUE) { Qnil, NULL };
+    VALUE obj = BigDecimal_allocate(1);
+    Real *vp = VpPtr(obj);
+    if (sign == VP_SIGN_NaN) {
+        VpSetNaN(vp);
+    }
+    else {
+        VpSetInf(vp, sign);
+    }
+    return (NULLABLE_BDVALUE) { obj, vp };
 }
 
 /*