public static string FixXmlHash(string str, uint target_hash)
{
str = Utils.StripBOM(str);
str = Utils.NormalizeLineEndings(str);
str += $"\n{XML_MARKER}\n<!-- Fix hash: ";
str += new CBDataHasher(str).CalculatePreimage(target_hash, " -->\n");
return(str);
}
/// <summary>
/// Finds a string that when appended to another string with the current hash, will have a final
/// hash equal to targetHash.
///
/// Hence, to bring the hash of a string to any value, do:
///
/// str += new CBDataHasher(str).CalculatePreimage(targetHash, " whatever text ");
/// </summary>
/// <param name="targetHash">The target hash value.</param>
/// <param name="tail">An optional suffix that the output string must end with.</param>
/// <returns>The string to append.</returns>
public string CalculatePreimage(uint targetHash, string tail = "")
{
// The effect of appending n characters to a string with this hash algorithm is effectively:
//
// hash(str + chars) = hash(str) * 33^n.Length + hash(chars) mod 2^32
//
// We calculate the hash value `chars` would need to have to set the hash to `targetHash`,
// assuming it is 5 characters.
//
// In this way, we can use an algorithm that only needs to bring 0 to a certain hash value,
// and not any value to any value.
CBDataHasher target = new CBDataHasher(targetHash);
target.UpdateReverse(tail); // account for the tail
target.State -= this.State * 39135393; // 33^5
// We calculate the string in reverse, effectively trying to bring the value of target down
// to zero. The calculation in reverse is:
//
// hash(c + str) = (hash(str) - c) * 33^1 mod 2^32
//
// As long as 33 evenly divides (hash(str) - c), the modular division will behave as ordinary
// division. As this value will always decrease, it eventually reaches zero, at which point
// we can just output null characters until we have 5 characters.
var outStr = "";
for (int i = 0; i < 5; i++)
{
var tint = target.State;
// Avoid generating surrogate pairs or BOMs (The CLR does weird things to it)
var next = (tint >= 0xD800 && tint <= 0xDFFF) || tint == 0xFEFF ? 0x752D + (tint % 33) :
tint <= 0xFFFF ? tint : 0xFFC0 + (tint % 33);
var next_ch = (char)next;
outStr = next_ch + outStr;
target.UpdateReverse(next_ch);
}
Trace.Assert(target.State == 0, "CalculatePreimage failed.");
return(outStr + tail);
}