// Copyright 2020 The Prometheus Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package labels

import (
	"regexp"
	"regexp/syntax"
	"strings"
)

type FastRegexMatcher struct {
	re       *regexp.Regexp
	prefix   string
	suffix   string
	contains string
}

func NewFastRegexMatcher(v string) (*FastRegexMatcher, error) {
	re, err := regexp.Compile("^(?:" + v + ")$")
	if err != nil {
		return nil, err
	}

	parsed, err := syntax.Parse(v, syntax.Perl)
	if err != nil {
		return nil, err
	}

	m := &FastRegexMatcher{
		re: re,
	}

	if parsed.Op == syntax.OpConcat {
		m.prefix, m.suffix, m.contains = optimizeConcatRegex(parsed)
	}

	return m, nil
}

func (m *FastRegexMatcher) MatchString(s string) bool {
	if m.prefix != "" && !strings.HasPrefix(s, m.prefix) {
		return false
	}
	if m.suffix != "" && !strings.HasSuffix(s, m.suffix) {
		return false
	}
	if m.contains != "" && !strings.Contains(s, m.contains) {
		return false
	}
	return m.re.MatchString(s)
}

func (m *FastRegexMatcher) GetRegexString() string {
	return m.re.String()
}

// optimizeConcatRegex returns literal prefix/suffix text that can be safely
// checked against the label value before running the regexp matcher.
func optimizeConcatRegex(r *syntax.Regexp) (prefix, suffix, contains string) {
	sub := r.Sub

	// We can safely remove begin and end text matchers respectively
	// at the beginning and end of the regexp.
	if len(sub) > 0 && sub[0].Op == syntax.OpBeginText {
		sub = sub[1:]
	}
	if len(sub) > 0 && sub[len(sub)-1].Op == syntax.OpEndText {
		sub = sub[:len(sub)-1]
	}

	if len(sub) == 0 {
		return
	}

	// Given Prometheus regex matchers are always anchored to the begin/end
	// of the text, if the first/last operations are literals, we can safely
	// treat them as prefix/suffix.
	if sub[0].Op == syntax.OpLiteral {
		prefix = string(sub[0].Rune)
	}
	if last := len(sub) - 1; sub[last].Op == syntax.OpLiteral {
		suffix = string(sub[last].Rune)
	}

	// If contains any literal which is not a prefix/suffix, we keep the
	// 1st one. We do not keep the whole list of literals to simplify the
	// fast path.
	for i := 1; i < len(sub)-1; i++ {
		if sub[i].Op == syntax.OpLiteral {
			contains = string(sub[i].Rune)
			break
		}
	}

	return
}
