interpres/fuzz_target_test.go

// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: MIT

package interpres

import (
	"context"
	"errors"
	"reflect"
	"strings"
	"testing"
	"time"
)

type fuzzNested struct {
	X int    `toml:"x"`
	Y string `toml:"y"`
}

type fuzzDoc struct {
	Num   int            `toml:"num"`
	Flt   float64        `toml:"flt"`
	Str   string         `toml:"str"`
	Flag  bool           `toml:"flag"`
	Small uint8          `toml:"small"`
	When  time.Time      `toml:"when"`
	Tags  []string       `toml:"tags"`
	Lims  map[string]any `toml:"lims"`
	Tab   fuzzNested     `toml:"tab"`
	Arr   []fuzzNested   `toml:"arr"`
	Other string         `toml:"other"`
}

// fuzzStmts is the statement pool the generated documents draw from: every
// destination kind the targeted parse handles, beside the shapes that make
// it fall back (overflow, unknown tables, duplicate keys).
var fuzzStmts = []string{
	`num = 1`, `num = 300`, `small = 300`, `small = 7`,
	`flt = 2.5`, `str = "x"`, `flag = true`,
	`when = 1979-05-27T07:32:00Z`,
	`tags = ["a", "b"]`, `tags = []`, `lims = { k = 1 }`,
	`[tab]`, `tab.x = 1`, `tab.y = "s"`, `x = 2`, `y = "t"`,
	`[[arr]]`, `x = 3`, `y = "u"`,
	`[tab.nested]`, `x = 4`,
	`other = "o"`, `zz = 1`, `[zz]`, `k = 1`,
	`num = 2`,
}

func fuzzDocument(data []byte) []byte {
	var b strings.Builder
	for i, by := range data {
		if i > 0 {
			b.WriteByte('\n')
		}
		b.WriteString(fuzzStmts[int(by)%len(fuzzStmts)])
	}
	return []byte(b.String())
}

// treeDecodeInto is the reference decode: the ordinary tree path, non-strict
// like the fuzz decode; the strict contracts have their own deterministic
// tests.
func treeDecodeInto(data []byte, v any) error {
	dec := newDecoder()
	tree, _, err := parseWithOptions(context.Background(), data, parseOptions{}, false)
	if err != nil {
		return err
	}
	return dec.decode(tree, v)
}

// decodeFinding normalises an error for the comparison. Decode-stage
// findings several tables may produce (an unknown field, a missing required
// key) compare as their class alone: the tree decode picks the reporting
// table by map order and so does not promise one. Everything else compares
// as its exact text.
func decodeFinding(err error) string {
	if err == nil {
		return ""
	}
	if de, ok := errors.AsType[*DecodeError](err); ok {
		if strings.Contains(de.Err.Error(), "unknown field") {
			return "unknown"
		}
		if strings.Contains(de.Err.Error(), "missing required key") {
			return "required"
		}
		return de.Path.String() + ": " + de.Err.Error()
	}
	return err.Error()
}

// FuzzTargetedDecode holds the targeted parse to the tree decode as its
// reference: for every generated document the two paths must agree on the
// error class and on the decoded value.
func FuzzTargetedDecode(f *testing.F) {
	seeds := []string{
		"num = 1\nstr = \"x\"\n[tab]\nx = 2\n[[arr]]\nx = 3\n",
		"small = 300\n",
		"[tab]\ntab.x = 1\n",
		"lims = { k = 1 }\ntags = [\"a\"]\n",
		"[[arr]]\ny = \"u\"\n[zz]\nk = 1\n",
		"when = 07:32:00\n[tab.nested]\n",
		"small = 300\n[[arr]]\nflt = 2.5\n",
	}
	for _, s := range seeds {
		f.Add([]byte(s))
	}
	f.Fuzz(func(t *testing.T, data []byte) {
		doc := fuzzDocument(data)
		var tgt fuzzDoc
		tgtErr := Unmarshal(doc, &tgt)
		if tgtErr != nil {
			// A document with several decode-stage findings reports a different
			// one per run (the tree decode walks its maps in random order), so the
			// reference gets a few chances to produce the finding the targeted
			// side carries. The targeted error is either the tree's own or the
			// fallback already reran the tree.
			for i := range 8 {
				var ref fuzzDoc
				refErr := treeDecodeInto(doc, &ref)
				if refErr == nil {
					t.Fatalf("reference succeeded on retry %d, targeted failed: %v\ndoc:\n%s", i, tgtErr, doc)
				}
				if decodeFinding(refErr) == decodeFinding(tgtErr) {
					return
				}
				if i == 7 {
					t.Fatalf("errors disagree after retries:\ntargeted: %v\nlast tree: %v\ndoc:\n%s", tgtErr, refErr, doc)
				}
			}
		}
		var ref fuzzDoc
		refErr := treeDecodeInto(doc, &ref)
		if refErr != nil {
			t.Fatalf("reference failed, targeted succeeded: %v\ndoc:\n%s", refErr, doc)
		}
		if !reflect.DeepEqual(ref, tgt) {
			t.Fatalf("values disagree:\ntree:     %#v\ntargeted: %#v\ndoc:\n%s", ref, tgt, doc)
		}
	})
}