parseFloat.zig

const std = @import("std");
const stdin = std.io.getStdIn().reader();
const stdout = std.io.getStdOut().writer();

const ParseError = error{Fail};

var index: usize = 0;
fn nextChar(buf: []const u8) u8 {
    defer index += 1;
    return buf[index];
}

fn isDigit(ch: u8) bool {
    const code = @as(i8, @intCast(ch));
    return code - '0' < 10 and code - '0' >= 0;
}

fn parseFloatingPoint(buf: []const u8) !f32 {
    var b: i32 = 0;
    var e: i32 = 0;
    var i: i32 = 0;
    var sign: u8 = '+';
    var ch = nextChar(buf);

    blk: {
        if (ch == '.') {
            ch = nextChar(buf);
            if (isDigit(ch)) {}
            b = ch - '0';
            e = 1;
        } else if (isDigit(ch)) {
            b = 10 * b + ch - '0';
            ch = nextChar(buf);

            while (isDigit(ch)) {
                b = 10 * b + ch - '0';
                ch = nextChar(buf);
            }
            if (ch == 'E') break :blk;
            if (ch != '.') return ParseError.Fail;
        } else {
            return ParseError.Fail;
        }

        ch = nextChar(buf);

        while (isDigit(ch)) {
            b = 10 * b + ch - '0';
            e += 1;
            ch = nextChar(buf);
        }

        if (ch != 'E') break :blk;
    }

    blk: {
        ch = nextChar(buf);
        if (ch == '+' or ch == '-') {
            sign = ch;
            ch = nextChar(buf);
        } else break :blk;

        while (isDigit(ch)) {
            i = 10 * i + ch - '0';
            ch = nextChar(buf);
        }
    }

    const sign_num: i32 = if (sign == '+') 1 else -1;
    return @as(f32, @floatFromInt(b)) * std.math.pow(f32, 10, @floatFromInt(sign_num * i - e));
}

pub fn main() !void {
    var buf = [_]u8{0} ** 0x100;

    try stdout.print("input floating point number >> ", .{});
    _ = try stdin.readUntilDelimiter(&buf, '\n');
    try stdout.print("accepted: {}\n", .{try parseFloatingPoint(&buf)});
}

test "digit" {
    const digits = [_]u8{ '0', '1', '2', '3', '4', '5', '6', '7', '8', '9' };

    for (digits) |digit|
        try std.testing.expect(isDigit(digit));

    try std.testing.expect(!isDigit(':'));
    try std.testing.expect(!isDigit('/'));
}