add a 'readable' format for large hex values

libdevcore/StringUtils.h

@@ -26,6 +26,8 @@
 #include <string>
 #include <vector>
 
+#include <libdevcore/CommonData.h>
+
 namespace dev
 {
 
@@ -72,4 +74,84 @@ std::string joinHumanReadable
 	return result;
 }
 
+/// Formats large numbers to be easily readable by humans.
+/// Returns decimal representation for smaller numbers; hex for large numbers.
+/// "Special" numbers, powers-of-two and powers-of-two minus 1, are returned in
+/// formulaic form like 0x01 * 2**24 - 1.
+/// @a T will typically by unsigned, u160, u256 or bigint.
+/// @param _value to be formatted
+/// @param _useTruncation if true, internal truncation is also applied,
+/// like  0x5555...{+56 more}...5555
+/// @example formatNumber((u256)0x7ffffff)
+template <class T>
+inline std::string formatNumberReadable(
+	T const& _value,
+	bool _useTruncation = false
+)
+{
+	static_assert(
+		std::is_same<bigint, T>::value || !std::numeric_limits<T>::is_signed,
+		"only unsigned types or bigint supported"
+	); //bigint does not carry sign bit on shift
+
+	// smaller numbers return as decimal
+	if (_value <= 0x1000000)
+		return _value.str();
+
+	HexCase hexcase = HexCase::Mixed;
+	HexPrefix prefix = HexPrefix::Add;
+
+	// when multiple trailing zero bytes, format as N * 2**x
+	int i = 0;
+	T v = _value;
+	for (; (v & 0xff) == 0; v >>= 8)
+		++i;
+	if (i > 2)
+	{
+		// 0x100 yields 2**8 (N is 1 and redundant)
+		if (v == 1)
+			return "2**" + std::to_string(i * 8);
+		return toHex(toCompactBigEndian(v), 2, prefix, hexcase) +
+			" * 2**" +
+			std::to_string(i * 8);
+	}
+
+	// when multiple trailing FF bytes, format as N * 2**x - 1
+	i = 0;
+	for (v = _value; (v & 0xff) == 0xff; v >>= 8)
+		++i;
+	if (i > 2)
+	{
+		// 0xFF yields 2**8 - 1 (v is 0 in that case)
+		if (v == 0)
+			return "2**" + std::to_string(i * 8) + " - 1";
+		return toHex(toCompactBigEndian(T(v + 1)), 2, prefix, hexcase) +
+			" * 2**" + std::to_string(i * 8) +
+			" - 1";
+	}
+
+	std::string str = toHex(toCompactBigEndian(_value), 2, prefix, hexcase);
+	if (_useTruncation)
+	{
+		// return as interior-truncated hex.
+		int len = str.size();
+
+		if (len < 24)
+			return str;
+
+		const int initialChars = (prefix == HexPrefix::Add) ? 6 : 4;
+		const int finalChars = 4;
+		int numSkipped = len - initialChars - finalChars;
+
+		return str.substr(0, initialChars) +
+			"...{+" +
+			std::to_string(numSkipped) +
+			" more}..." +
+			str.substr(len-finalChars, len);
+	}
+
+	// otherwise, show whole value.
+	return str;
+}
+
 }