tm.jl

# run like this:
# julia -O3 --check-bounds=no tm.jl busy-beaver-5.json --fast
using ArgParse
using JSON

# ANSI codes for inverse video and reset
const INVERSE = "\u001B[7m"
const RESET = "\u001B[0m"

function parse_commandline()
    s = ArgParseSettings(
        prog = "tm",
        version = "0.1.0",
        description = "A Turing Machine simulator that loads a machine from JSON"
    )

    @add_arg_table! s begin
        "--fast"
            help = "if present, only print every 100k steps (faster)"
            action = "store_true"
        "filename"
            help = "the JSON file describing the Turing Machine"
            required = true
    end

    return parse_args(s)
end

# Converts a JSON value to string, removing surrounding quotes
function to_str(val)
    if val isa String
        return val
    else
        return string(val)
    end
end

function main()
    # Parse command line arguments
    args = parse_commandline()

    # Read and parse JSON file
    text = try
        read(args["filename"], String)
    catch e
        println(stderr, "Error reading file $(args["filename"]): $e")
        exit(1)
    end

    tm = try
        JSON.parse(text)
    catch e
        println(stderr, "Error parsing JSON in $(args["filename"]): $e")
        exit(1)
    end

    # Mandatory fields: "blank" & "initial"
    blank = get(tm, "blank", nothing)
    initial = get(tm, "initial", nothing)

    if isnothing(blank) || isnothing(initial)
        println(stderr, "Error: JSON missing mandatory fields 'blank' or 'initial'.")
        exit(1)
    end

    # Optional fields
    name = get(tm, "name", nothing)
    desc = get(tm, "description", nothing)
    author = get(tm, "author", nothing)
    date = get(tm, "date", nothing)
    link = get(tm, "link", nothing)
    paper = get(tm, "paper", nothing)
    tape_str = get(tm, "tape", "")
    position = get(tm, "position", 0)

    # Build transitions dictionary
    transitions = Dict{Tuple{String,String}, Tuple{String,String,String}}()
    if haskey(tm, "transitions") && tm["transitions"] isa Vector
        for rule in tm["transitions"]
            if rule isa Vector && length(rule) == 5
                old_state = to_str(rule[1])
                old_symbol = to_str(rule[2])
                new_symbol = to_str(rule[3])
                direction = uppercase(to_str(rule[4]))
                new_state = to_str(rule[5])
                transitions[(old_state, old_symbol)] = (new_symbol, direction, new_state)
            end
        end
    end

    # Create initial tape
    tape = isempty(tape_str) ? [string(blank)] : [string(c) for c in tape_str]

    # Position fix
    while position < 0
        pushfirst!(tape, string(blank))
        position += 1
    end
    while position >= length(tape)
        push!(tape, string(blank))
    end

    # Print summary
    println("\nStarting Turing machine")
    !isnothing(name) && println("Name: $name")
    !isnothing(desc) && println("Description: $desc")
    !isnothing(author) && println("Author: $author")
    !isnothing(date) && println("Date: $date")
    !isnothing(link) && println("Link: $link")
    !isnothing(paper) && println("Paper: $paper")
    println("(blank='$blank', initial='$initial', position=$position)\n")

    # Run the Turing machine
    fast_mode = args["fast"]
    step = 0
    state = string(initial)

    while true
        do_print = !fast_mode || step % 100_000 == 0
        if do_print
            joined_tape = join([i == position + 1 ? INVERSE * sym * RESET : sym for (i, sym) in enumerate(tape)])
            println("Step $(lpad(step, 6))  State=$(rpad(state, 8)) Pos=$(lpad(position, 5)) Tape=$joined_tape")
        end

        # Current symbol
        current_symbol = tape[position + 1]  # Julia is 1-based

        # Find rule
        key = (state, current_symbol)
        rule = get(transitions, key, nothing)
        if isnothing(rule)
            rule = get(transitions, (state, "*"), nothing)
            if !isnothing(rule)
                new_symbol, direction, new_state = rule
                new_symbol = new_symbol == "*" ? current_symbol : new_symbol
            else
                println("\nHalted: no rule for state=$state and symbol='$current_symbol'")
                break
            end
        else
            new_symbol, direction, new_state = rule
            new_symbol = new_symbol == "*" ? current_symbol : new_symbol
        end

        tape[position + 1] = new_symbol
        state = new_state

        # Handle movement
        if direction == "L"
            position -= 1
            if position < 0
                pushfirst!(tape, string(blank))
                position = 0
            end
        elseif direction == "R"
            position += 1
            while position >= length(tape)
                push!(tape, string(blank))
            end
        end

        step += 1
    end

    # Final tape
    final_tape = join(tape)
    while startswith(final_tape, blank)
        final_tape = final_tape[length(blank)+1:end]
    end
    while endswith(final_tape, blank)
        final_tape = final_tape[1:end-length(blank)]
    end

    println("\nFinal tape: $final_tape")
    println("Steps: $step")

    # Print histogram
    histogram = Dict{String,Int}()
    for sym in tape
        histogram[sym] = get(histogram, sym, 0) + 1
    end
    println("Histogram:")
    for (sym, count) in histogram
        println("  symbol: $sym, count: $count")
    end
end

main()