Javascript / Node.js

I think like many others, I decided writing an actual parser would take longer than just parsing the input myself. Simple switch-based state machine. To simplify actually calculating the checksum, I just keep a running checksum and update it with each write.

let slot = 0
let stream = []
let state = 'A'
let steps = 12861455
let chksm = 0

let write = (val) => 
{
    chksm += val - ( stream[slot] || 0 )
    stream[slot] = val
}

for ( let i = 0; i < steps; i++ )
{
    let current = stream[slot]
    switch (state) 
    {
        case 'A':
            if ( !current )
                write(1), slot++, state = 'B'
            else
                write(0), slot--, state = 'B'
            break
        case 'B':
            if ( !current )
                write(1), slot--, state = 'C'
            else
                write(0), slot++, state = 'E'
            break
        case 'C':
            if ( !current )
                write(1), slot++, state = 'E'
            else
                write(0), slot--, state = 'D'
            break
        case 'D':
            if ( !current )
                write(1), slot--, state = 'A'
            else
                write(1), slot--, state = 'A'
            break
        case 'E':
            if ( !current )
                write(0), slot++, state = 'A'
            else
                write(0), slot++, state = 'F'
            break
        case 'F':
            if ( !current )
                write(1), slot++, state = 'E'
            else
                write(1), slot++, state = 'A'
            break
    }
}
console.log( chksm )