// Package mastosan takes Mastodon flavored HTML and emits Slack-flavored
// markdown.
//
// It works by using an embedded Rust program compiled to WebAssembly. At
// the time of writing this adds an extra 0.2 seconds to compile and run the
// WebAssembly module, but this can probably be fixed with aggressive sync.Once
// caching should this prove a problem in the real world.
//
// Building this Rust program outside of Nix flakes is UNSUPPORTED.
//
// To build the Rust module from inside the Nix flake:
//
//     cargo install wasm-snip
//     cargo build --release
//     wasm-opt -Oz -o ./testdata/mastosan-pre-snip.wasm
//     wasm-snip --skip-producers-section --snip-rust-panicking-code --snip-rust-fmt-code -i ./testdata/mastosan-pre-snip.wasm ./testdata/mastosan.wasm
//     rm ./testdata/mastosan-pre-snip.wasm
//
// This adds about two megabytes to the resulting binary, including the AOT
// WebAssembly runtime wazero: https://wazero.io/
package mastosan

import (
	"bytes"
	"context"
	_ "embed"
	"log"
	"math/rand"
	"strconv"
	"strings"

	"github.com/tetratelabs/wazero"
	"github.com/tetratelabs/wazero/imports/wasi_snapshot_preview1"
)

var (
	//go:embed testdata/mastosan.wasm
	mastosanWasm []byte

	r    wazero.Runtime
	code wazero.CompiledModule
)

func init() {
	ctx := context.Background()
	r = wazero.NewRuntime(ctx)

	wasi_snapshot_preview1.MustInstantiate(ctx, r)

	var err error
	code, err = r.CompileModule(ctx, mastosanWasm)
	if err != nil {
		log.Panicln(err)
	}
}

func runWASM(ctx context.Context, text, mode string) (string, error) {
	fout := &bytes.Buffer{}
	fin := bytes.NewBufferString(text)

	name := strconv.Itoa(rand.Int())
	config := wazero.NewModuleConfig().WithStdout(fout).WithStdin(fin).WithArgs("mastosan", mode).WithName(name)

	mod, err := r.InstantiateModule(ctx, code, config)
	if err != nil {
		return "", err
	}
	defer mod.Close(ctx)

	return strings.TrimSpace(fout.String()), nil
}

// Slackdown converts a string full of HTML text to slack-flavored markdown.
//
// Internally this works by taking that HTML and piping it to a small Rust program
// using lol_html to parse the HTML and rejigger it into slack-flavored markdown.
func Slackdown(ctx context.Context, text string) (string, error) {
	return runWASM(ctx, text, "slackdown")
}

// Text converts a string with HTML content into an approximation of plain text.
//
// Internally this works by taking that HTML and piping it to a small Rust program
// using lol_html to parse the HTML and rejigger it into plain text.
func Text(ctx context.Context, text string) (string, error) {
	return runWASM(ctx, text, "text")
}

// Markdown converts a string with HTML content into an approximation of plain text.
//
// Internally this works by taking that HTML and piping it to a small Rust program
// using lol_html to parse the HTML and rejigger it into generic markdown.
func Markdown(ctx context.Context, text string) (string, error) {
	return runWASM(ctx, text, "markdown")
}