# Useful Data Structures ------------------------------------------------------

# Basically just an enum for `Axis` types
abstract type Axis end
struct XAxis <: Axis end
struct YAxis <: Axis end

struct Fold
    axis::Axis
    index::Int
end

mutable struct Paper
    dots::BitMatrix
    folds::Vector{Fold}
end


# Ingest the Data -------------------------------------------------------------

function ingest(path)
    return open(path) do f

        # Start by reading all the lines from the top section of the input file
        # and parsing each one into a `CartesianIndex`.
        coordinateslist = []
        for coordinatestring in split(readuntil(f, "\n\n"))
            (col, row) = [parse(Int, s) for s in split(coordinatestring, ",")]
            push!(coordinateslist, CartesianIndex(row+1, col+1))
        end

        # Next read in the lines from the bottom section and parse each
        # one into a `Fold` struct.
        folds = []
        foldre = r"(x|y)=(\d+)"
        for foldstring in split(readchomp(f), "\n")
            m = match(foldre, foldstring)
            axis = m[1] == "x" ? XAxis() : YAxis()
            index = parse(Int, m[2]) + 1
            fold = Fold(axis, index)
            push!(folds, fold)
        end

        # Figure out how many rows/columns the `dots` Matrix should be. We
        # assume that it must be tall and wide enough to accommodate the
        # largest horizontal and vertical fold.
        rows = cols = 0
        for fold in folds
            if isa(fold.axis, XAxis)
                cols = max(cols, (fold.index * 2) - 1) 
            else
                rows = max(rows, (fold.index * 2) - 1) 
            end
        end

        # Make a large enough BitMatrix to accommodate all the 'dots' and
        # and set each coordinate found above to `true`.
        dots = falses(rows, cols)
        for coordinate in coordinateslist
            dots[coordinate] = true
        end

        Paper(dots, folds)
    end
end

# Given a BitMatrix representing the current arrangement of dots on a page
# and a fold indicating where/how to fold that paper, fold the paper and
# return the result.
function foldit(dots::BitMatrix, fold::Fold)::BitMatrix
    (rows, cols) = size(dots)

    # Need to define two views into the `dots` BitMatrix, one for the 
    # half of the paper that will stay in place, and one for the half
    # of the paper to be 'folded over'. The 'folded' view will have
    # its rows/columns reversed as appropriate.
    toprows  = bottomrows = 1:rows
    leftcols = rightcols  = 1:cols
    if isa(fold.axis, XAxis)
        leftcols   = 1:(fold.index-1)
        rightcols = cols:-1:(fold.index+1)
    else
        toprows    = 1:(fold.index-1)
        bottomrows = rows:-1:(fold.index+1)
    end

    # Take the two views, overlay them, then return the result
    still  = view(dots, toprows,    leftcols) # Stationary!
    folded = view(dots, bottomrows, rightcols)
    return (still .| folded)
end

# Take the input, fold it once, then return the number of 'dots' from
# that single fold.
function part1(input)
    dots = input.dots
    fold = input.folds[1]
    folded = foldit(dots, fold)
    return count(folded)
end

# Iteration for a `Paper` Struct ----------------------------------------------
struct PaperFoldState
    dots::BitMatrix
    lastfold::Int
end

# Initial iterator, returns the first fold
function Base.iterate(paper::Paper)
    fold = paper.folds[1]
    folded = foldit(paper.dots, fold)
    state = PaperFoldState(folded, 1)
    return (folded, state)
end

# Subsequent iterator, given the last state, return tne next state
function Base.iterate(paper::Paper, state::PaperFoldState)
    (dots, lastfold) = (state.dots, state.lastfold)
    lastfold >= length(paper.folds) && return nothing
    fold = paper.folds[lastfold + 1]
    folded = foldit(dots, fold)
    state = PaperFoldState(folded, lastfold + 1)
    return (folded, state)
end

# Needed to be able to get a specific fold state of the `Paper`
function Base.getindex(paper::Paper, i::Int)
    for (iteration, dots) in enumerate(paper)
        iteration > i && return dots
    end
    throw(BoundsError(paper, i))
end

# Some more necessary implementations so I can use the `paper[end]` syntax
# in our solution function.
Base.length(paper::Paper)    = length(paper.folds) - 1
Base.lastindex(paper::Paper) = lastindex(paper.folds) - 1
Base.eltype(::Type{Paper})   = BitMatrix

# Pretty prints a BitMatrix to make the solution to Part Two more
# readable, because reading the block characters from the default
# 1/0 display of the BitMatrix is difficult.
function prettyprint(matrix::BitMatrix)
    for line in eachrow(matrix)
        for bit in line
            if bit; print('█'); else; print(' '); end
        end
        println()
    end
end


# Solve for Part Two ----------------------------------------------------------

# Fold the paper until all the folds are used up. The commented out print
# statement is there for solving the puzzle. The rest of it is there for 
# comparing in my test suite.
function part2(input)
    final_paper = input[end]
    # prettyprint(final_paper)
    rowsums = mapslices(sum, final_paper, dims = 2)
    colsums = mapslices(sum, final_paper, dims = 1)
    return (vec(rowsums), vec(colsums))
end