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

package base

import (
	"errors"
	"strings"
	"testing"
)

// TestWrapErrCarriesTagOnce pins the WrapErr contract the ten wrap
// points rely on: the message carries the tensor tag and the operation
// once each, whatever the inner error already carries, and the chain
// stays open for errors.Is. The wrap these properties answer to used to
// be fmt.Errorf("op: %w", err) over an Errf-built cause, which printed
// the tag twice.
func TestWrapErrCarriesTagOnce(t *testing.T) {
	if got := WrapErr("Anything", nil); got != nil {
		t.Fatalf("WrapErr of nil = %v, want nil", got)
	}

	// A bare cause the library did not build: the tag and the operation
	// are added, once each.
	bare := errors.New("the caller's own failure")
	got := WrapErr("Jacobian", bare)
	if want := "tensor: Jacobian: the caller's own failure"; got.Error() != want {
		t.Errorf("WrapErr of a bare cause = %q, want %q", got.Error(), want)
	}
	if !errors.Is(got, bare) {
		t.Errorf("WrapErr of a bare cause does not unwrap to it")
	}

	// A cause the library already tagged: the tag is not doubled.
	tagged := Errf("the shape holds more elements than fit in an index")
	got = WrapErr("Tile", tagged)
	if want := "tensor: Tile: the shape holds more elements than fit in an index"; got.Error() != want {
		t.Errorf("WrapErr of a tagged cause = %q, want %q", got.Error(), want)
	}
	if !errors.Is(got, tagged) {
		t.Errorf("WrapErr of a tagged cause does not unwrap to it")
	}
	if n := strings.Count(got.Error(), "tensor: "); n != 1 {
		t.Errorf("WrapErr message carries the tag %d times, want exactly one", n)
	}

	// A nested WrapErr composes the same way: each layer names its own
	// operation and the tag still appears once.
	inner := WrapErr("Concat", tagged)
	got = WrapErr("Repeat", inner)
	if want := "tensor: Repeat: Concat: the shape holds more elements than fit in an index"; got.Error() != want {
		t.Errorf("nested WrapErr = %q, want %q", got.Error(), want)
	}
	if !errors.Is(got, tagged) {
		t.Errorf("nested WrapErr does not unwrap down to the cause")
	}
}