Optimize codes

Force fileGet to use binary mode to prevent some problem.
Make cache system serve the existing cache when download failed.
Replace some if-statements with switch-statements.
Clean up codes.

src/string_hash.h

@@ -0,0 +1,38 @@
+#ifndef STRING_HASH_H_INCLUDED
+#define STRING_HASH_H_INCLUDED
+
+#include <string>
+
+typedef uint64_t hash_t;
+
+constexpr hash_t prime = 0x100000001B3ull;
+constexpr hash_t basis = 0xCBF29CE484222325ull;
+
+hash_t hash_(char const* str)
+{
+    hash_t ret{basis};
+    while(*str)
+    {
+        ret ^= *str;
+        ret *= prime;
+        str++;
+    }
+    return ret;
+}
+
+hash_t hash_(const std::string &str)
+{
+    return hash_(str.data());
+}
+
+constexpr hash_t hash_compile_time(char const* str, hash_t last_value = basis)
+{
+    return *str ? hash_compile_time(str + 1, (*str ^ last_value) * prime) : last_value;
+}
+
+constexpr unsigned long long operator "" _hash(char const* p, size_t)
+{
+    return hash_compile_time(p);
+}
+
+#endif // STRING_HASH_H_INCLUDED