added fast_istr for fast int_to_string

include/c11log/details/fast_istostr.h

+#pragma once
+#include <string>
+
+//Fast to int to string
+//Source: http://stackoverflow.com/a/4351484/192001
+//Modified version to pad zeros according to min_size
+
+namespace c11log {
+namespace details {
+
+const char digit_pairs[201] = {
+    "00010203040506070809"
+    "10111213141516171819"
+    "20212223242526272829"
+    "30313233343536373839"
+    "40414243444546474849"
+    "50515253545556575859"
+    "60616263646566676869"
+    "70717273747576777879"
+    "80818283848586878889"
+    "90919293949596979899"
+};
+
+
+inline std::string& fast_itostr(int n, std::string& s, int padding)
+{
+    if (n == 0)
+    {
+        s = "0";
+        return s;
+    }
+
+    int sign = -(n < 0);
+    unsigned int val = (n^sign) - sign;
+
+    int size;
+    if (val >= 10000)
+    {
+        if (val >= 10000000)
+        {
+            if (val >= 1000000000)
+                size = 10;
+            else if (val >= 100000000)
+                size = 9;
+            else
+                size = 8;
+        }
+        else
+        {
+            if (val >= 1000000)
+                size = 7;
+            else if (val >= 100000)
+                size = 6;
+            else
+                size = 5;
+        }
+    }
+    else
+    {
+        if (val >= 100)
+        {
+            if (val >= 1000)
+                size = 4;
+            else
+                size = 3;
+        }
+        else
+        {
+            if (val >= 10)
+                size = 2;
+            else
+                size = 1;
+        }
+    }
+    size -= sign;
+    if (size < padding)
+        size = padding;
+
+    s.resize(size);
+    char* c = &s[0];
+    if (sign)
+        *c = '-';
+
+    c += size - 1;
+    while (val >= 100)
+    {
+        int pos = val % 100;
+        val /= 100;
+        *(short*)(c - 1) = *(short*)(digit_pairs + 2 * pos);
+        c -= 2;
+    }
+    while (val > 0)
+    {
+        *c-- = '0' + (val % 10);
+        val /= 10;
+    }
+
+    while (c >= s.data())
+        *c-- = '0';
+    return s;
+}
+}
+}
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