1 files changed, 148 insertions, 0 deletions
diff --git a/lib/strtoull.c b/lib/strtoull.c
new file mode 100644
index 0000000..bb71c24
--- /dev/null
+++ b/lib/strtoull.c
@@ -0,0 +1,148 @@
+#include <stdbool.h>
+
+static long long valueof(int c, int base)
+{
+    long long result = 0;
+
+    if (base < 2 || base > 36)
+    {
+        // bases less than 2 or greater than 36 are invalid.
+        return -1;
+    }
+
+    // check if value is an ascii numeric character
+    if (c >= '0' && c <= '9')
+    {
+        // simple as
+        result = (long long)(c ^ 0x30);
+    }
+
+    // check if value is an uppercase ASCII alphabetical character
+    else if (
+            (c >= 'A' && c <= 'Z') ||
+            (c >= 'a' && c <= 'z'))
+    {
+        // mask in uppercase bit
+        c |= 0x20;
+        // value is 10 plus the alphabetical order of the character
+        result = (long long)(10 + c - 'a');
+    }
+    else
+    {
+        // character has no valid interpretation under base 36
+        return -1LL;
+    }
+
+    if (result > (base - 1))
+    {
+        // character is not valid under given base
+        return -1LL;
+    }
+
+    return result;
+}
+
+unsigned long long strtoull(
+        const char *restrict begin,
+        char **restrict end,
+        int base)
+{
+    long result = 0;
+    const char *current = begin;
+
+    // check for empty string
+    if (*current == 0)
+    {
+        if (end)
+        {
+            *end = (char *)begin;
+            return result;
+        }
+    }
+
+    // skip initial whitespace
+    bool whitespace = true;
+    while (whitespace)
+    {
+        switch (*current)
+        {
+            case 0x9:
+            case 0xa:
+            case 0xb:
+            case 0xc:
+            case 0xd:
+            case 0x20:
+                current++;
+                break;
+            default:
+                whitespace = false;
+        }
+    }
+
+    bool negative = false;
+    // detect if negative sign is used
+    if (*current == '-')
+    {
+        negative = true;
+        current++;
+    }
+
+    // detect base from input
+    if (base == 0)
+    {
+        // base is non-decimal
+        if (current[0] == 0)
+        {
+            if (current[1] == 'b')
+            {
+                base = 2;
+                current += 2;
+            }
+            else if (current[1] == 'x')
+            {
+                base = 16;
+                current += 2;
+            }
+            else
+            {
+                base = 8;
+                current++;
+            }
+        }
+        // base is decimal otherwise
+        else
+        {
+            base = 10;
+        }
+    }
+    // do the actual conversion now
+    while (*current)
+    {
+        long long value = valueof(*current, base);
+
+        // we've reached the end of the conversion
+        if (value == -1)
+        {
+            break;
+        }
+
+        result *= base;
+        result += value;
+        current++;
+    }
+
+    // store the end pointer if needed
+    if (end)
+    {
+        *end = (char *)current;
+    }
+
+    // flip the result if we're meant to
+    if (negative)
+    {
+        result = -result;
+    }
+
+    return result;
+}
+