feat: add LeetCode problem 65

Solution for valid number

leetcode/src/65.c

@@ -0,0 +1,83 @@
+/*
+ * Returns a boolean value of true if the input string is a valid number.
+ * Heavily optimized solution that utilizes branch prediction builtin's provided by
+ * GCC and clang.
+ * Uses a 4-bit bitset to track state of the program.
+ *
+ * Copyright (C) Mustafa Aamir <mustafa.290101@gmail.com>
+ * */
+
+
+/* Defining branch prediction and inline macros */
+#if defined(__GNUC__) || defined(__clang__)
+#define likely(x) __builtin_expect(!!(x), 1)
+#define unlikely(x) __builtin_expect(!!(x), 0)
+#define _force_inline __attribute__((always_inline)) static inline
+#else
+#define likely(x) x
+#define unlikely(x) x
+#define _force_inline static inline
+#endif
+
+/* stdbool compatibility */
+#ifndef __STDBOOL_H
+#define bool int
+#define true 1
+#define false 0
+#endif
+
+
+/* Bitset to track the state of the program
+ * Going from MSB to LSB, a high value for each bit represents the following values:
+ * 1. decimal: '.' used
+ * 2. sign '+' or '-' seen
+ * 3. exponential 'E' or 'e' seen
+ * 4. Integer value {0..9} seen
+ */
+typedef struct {
+    unsigned flag : 4;
+} state_t;
+
+_force_inline bool isDigit(const char c) {
+    return (unsigned char)(c - '0') < 10;
+}
+
+_force_inline bool isNumber(const char *s) {
+    // initializing state bitset
+    state_t state = {0};
+
+    // performing checks on the first char of the input
+    if (unlikely(*s == '\0')) {
+        return false;
+    } else if (likely(isDigit(*s))) {
+        state.flag |= 0x1;
+    } else if (likely(*s == '+' || *s == '-')) {
+        state.flag |= 0x4;
+    } else if (*s == '.') {
+        state.flag |= 0x8;
+    } else {
+      return false;
+    }
+
+    // iterating over subsequent characters of the input string
+    while (likely(*++s != '\0')) {
+        if (likely(isDigit(*s))) {
+            state.flag |= 1;
+            continue;
+        }
+        switch (*s) {
+            case 'E': case 'e':
+                if (state.flag & 2 || !(state.flag & 1)) return false;
+                state.flag = 2; break;
+            case '+': case '-':
+                if (state.flag & 13) return false; // check for exp?
+                state.flag |= 4; break;
+            case '.':
+                if (unlikely(state.flag & 10)) return false;
+                state.flag |= 8; break;
+            default:
+                return false;
+        }
+    }
+    return state.flag & 1;
+}