# copied from Pkg.jl/src/GitTools.jl

module GitTools

using SHA
import Base: SHA1

@enum GitMode mode_dir=0o040000 mode_normal=0o100644 mode_executable=0o100755 mode_symlink=0o120000 mode_submodule=0o160000
Base.string(mode::GitMode) = string(UInt32(mode); base=8)
Base.print(io::IO, mode::GitMode) = print(io, string(mode))

function gitmode(path::AbstractString)
    # Windows doens't deal with executable permissions in quite the same way,
    # `stat()` gives a different answer than we actually want, so we use
    # `isexecutable()` which uses `uv_fs_access()` internally.  On other
    # platforms however, we just want to check via `stat()`.
    function isexec(p)
        @static if Sys.iswindows()
            return Sys.isexecutable(p)
        end
        return !iszero(filemode(p) & 0o100)
    end
    if islink(path)
        return mode_symlink
    elseif isdir(path)
        return mode_dir
    elseif isexec(path)
        return mode_executable
    else
        return mode_normal
    end
end

"""
    blob_hash(HashType::Type, path::AbstractString)

Calculate the git blob hash of a given path.
"""
function blob_hash(::Type{HashType}, path::AbstractString) where HashType
    ctx = HashType()
    if islink(path)
        datalen = length(readlink(path))
    else
        datalen = filesize(path)
    end

    # First, the header
    SHA.update!(ctx, Vector{UInt8}("blob $(datalen)\0"))

    # Next, read data in in chunks of 4KB
    buff = Vector{UInt8}(undef, 4*1024)

    try
        if islink(path)
            update!(ctx, Vector{UInt8}(readlink(path)))
        else
            open(path, "r") do io
                while !eof(io)
                    num_read = readbytes!(io, buff)
                    update!(ctx, buff, num_read)
                end
            end
        end
    catch e
        if isa(e, InterruptException)
            rethrow(e)
        end
        @warn("Unable to open $(path) for hashing; git-tree-sha1 likely suspect")
    end

    # Finish it off and return the digest!
    return SHA.digest!(ctx)
end
blob_hash(path::AbstractString) = blob_hash(SHA1_CTX, path)

"""
    contains_files(root::AbstractString)

Helper function to determine whether a directory contains files; e.g. it is a
direct parent of a file or it contains some other directory that itself is a
direct parent of a file. This is used to exclude directories from tree hashing.
"""
function contains_files(path::AbstractString)
    st = lstat(path)
    ispath(st) || throw(ArgumentError("non-existent path: $(repr(path))"))
    isdir(st) || return true
    for p in readdir(path)
        contains_files(joinpath(path, p)) && return true
    end
    return false
end


"""
    tree_hash(HashType::Type, root::AbstractString)

Calculate the git tree hash of a given path.
"""
function tree_hash(::Type{HashType}, root::AbstractString) where HashType
    entries = Tuple{String, Vector{UInt8}, GitMode}[]
    for f in readdir(root)
        # Skip `.git` directories
        if f == ".git"
            continue
        end

        filepath = abspath(root, f)
        mode = gitmode(filepath)
        if mode == mode_dir
            # If this directory contains no files, then skip it
            contains_files(filepath) || continue

            # Otherwise, hash it up!
            hash = tree_hash(HashType, filepath)
        else
            hash = blob_hash(HashType, filepath)
        end
        push!(entries, (f, hash, mode))
    end

    # Sort entries by name (with trailing slashes for directories)
    sort!(entries, by = ((name, hash, mode),) -> mode == mode_dir ? name*"/" : name)

    content_size = 0
    for (n, h, m) in entries
        content_size += ndigits(UInt32(m); base=8) + 1 + sizeof(n) + 1 + sizeof(h)
    end

    # Return the hash of these entries
    ctx = HashType()
    SHA.update!(ctx, Vector{UInt8}("tree $(content_size)\0"))
    for (name, hash, mode) in entries
        SHA.update!(ctx, Vector{UInt8}("$(mode) $(name)\0"))
        SHA.update!(ctx, hash)
    end
    return SHA.digest!(ctx)
end
tree_hash(root::AbstractString) = tree_hash(SHA.SHA1_CTX, root)

end # module
