Lesson 05 · Parsing sign and rejecting bad input - Build an Arbitrary-Precision Integer Library

Parsing sign and rejecting bad input

A real parser handles a leading minus, collapses a negative zero, and refuses garbage. Today you wrap the magnitude parser into the public Parse and pin its edges.

The goal

Parse an optional leading minus and digits into a BigInt, rejecting invalid input and canonicalizing "-0".

Start here - the target

TO DO

Scenario: Parse handles sign, leading zeros, negative zero, and invalid input

Giventhe string "-123"

WhenParse is called

Thenit succeeds with Sign() -1 and a magnitude of one limb equal to 123

AndParse("-0") succeeds with Sign() 0 and an empty magnitude, Parse("007") succeeds with Sign() 1 and limb 7, and Parse("") and Parse("12x") each return an error

Background

parseMag trusts its input; Parse does not. It peels an optional leading -, then requires that what remains is a non-empty run of decimal digits - an empty string, a lone -, or anything with a non-digit character is an error, not a zero. Leading zeros are fine ("007" is 7), because grouping and normalization handle them for free.

The one rule to hold onto is the canonical zero from lesson 3: if the parsed magnitude is empty, the sign must be 0 regardless of a leading minus, so "-0" parses to the same value as "0". Wire that check in here and negative zero can never enter the system through the front door.

Make it work

func Parse(s string) (BigInt, error) {
  neg := false
  if len(s) > 0 && s[0] == '-' { neg = true; s = s[1:] }
  // reject empty body or any non-digit rune here
  m := parseMag(s)
  sign := 1
  if len(m) == 0 { sign = 0 } else if neg { sign = -1 }
  return BigInt{sign, m}, nil
}

CheckpointDONE

Parse reads signed decimal input and rejects garbage. Commit and stop here.