@Override
public Success<Expression> parse(String s, int p) {
// Parse a string of digits before and after the decimal point.
Success<String> resA = parseDigits(s, p);
p = resA.rem;
if (s.charAt(p++) != '.') return null;
Success<String> resB = parseDigits(s, p);
p = resB.rem;
// Put them together.
String a = resA.value, b = resB.value;
if (a.isEmpty() && b.isEmpty()) return null;
StringBuilder sb = new StringBuilder();
sb.append(a).append('.').append(b);
// Parse the exponent.
if (Character.isLowerCase(s.charAt(p))) {
sb.append(s.charAt(p++));
if (s.charAt(p) == '-') {
++p;
sb.append('-');
}
Success<String> resExp = parseDigits(s, p);
if (resExp.value.isEmpty()) return null;
sb.append(resExp.value);
}
Expression Success = new LiteralDouble(Double.parseDouble(sb.toString()));
return new Success<Expression>(Success, p);
}
private static Success<String> parseDigits(String s, int p) {
StringBuilder sb = new StringBuilder();
while (Character.isDigit(s.charAt(p))) sb.append(s.charAt(p++));
return new Success<String>(sb.toString(), p);
}