filter/filter.go

// Copyright (C) 2017 Marius Schellenberger

// Package filter implements simple struct field filtering
package filter

import (
	"reflect"
	"strconv"
	"strings"
)

const any = '*'

type Filter interface {
	Len() int
	Index(int) interface{}
	Remove(int, int)
}

type match struct {
	fields []string
	value  string
}

// Query implements the filter query
type Query []match

// NewQuery parses the filter query.
// Examples:
// Explicit match: <struct field>:<struct value>
// Wildcard match:
// <struct field>:*<part of struct value>
// <struct field>:<part of struct value>*
// <struct field>:*<middl of struct value>*
// Nested structs:
// <struct field>.<struct field>:<struct value>
// Multiple matches:
// <struct field>:<struct value> <struct field>:<struct value>*
// Supported data types are: string, int, bool
// Bool does not support the wildcard operator
func NewQuery(s string) (q Query) {
	fields := strings.Split(s, " ")
	for _, v := range fields {
		m := strings.Split(v, ":")
		if len(m) == 2 {
			q = append(q, match{strings.Split(m[0], "."), m[1]})
		}
	}
	return
}

func (q Query) match(v reflect.Value) (ok bool) {
	if v.Kind() != reflect.Struct {
		return
	}
	if len(q) == 0 {
		return
	}
	ok = true
	for _, qv := range q {
		s := ""
		fl := len(qv.fields)
		if fl == 0 {
			continue
		}
		val := v.FieldByName(qv.fields[0])
		for i := 1; i < fl; i++ {
			val = val.FieldByName(qv.fields[i])
		}
		switch val.Kind() {
		case reflect.Int, reflect.Int8, reflect.Int16,
			reflect.Int32, reflect.Int64:
			s = strconv.Itoa(int(val.Int()))
		case reflect.String:
			s = val.String()
		case reflect.Bool:
			if val.Bool() {
				s = "true"
			} else {
				s = "false"
			}
			if s != qv.value {
				ok = false
			}
			continue
		default:
			ok = false
			continue
		}
		vl := len(qv.value)
		switch {
		case vl >= 3:
			if qv.value[0] == any && qv.value[vl-1] == any {
				if strings.Contains(s, qv.value[1:vl-1]) {
					continue
				}
			}
			fallthrough
		case vl >= 2:
			if qv.value[0] == any {
				if strings.HasSuffix(s, qv.value[1:]) {
					continue
				}
			}
			if qv.value[vl-1] == any {
				if strings.HasPrefix(s, qv.value[:vl-1]) {
					continue
				}
			}
		}
		if s != qv.value {
			ok = false
		}
	}
	return
}

// GenericFilter takes any slice of structs value and returns the same slice type containing only the filter matches.
// Returns nil if interface{} is not a slice.
func (q Query) GenericFilter(i interface{}) interface{} {
	v := reflect.ValueOf(i)
	if v.Kind() != reflect.Slice {
		return nil
	}
	ret := reflect.MakeSlice(v.Type(), 0, 0)
	for i := 0; i < v.Len(); i++ {
		val := v.Index(i)
		if q.match(val) {
			ret = reflect.Append(ret, val)
		}
	}
	return ret.Interface()
}

func (q Query) Filter(f Filter) {
	count := 0
	i := 0
	for ; i < f.Len(); i++ {
		if q.match(reflect.ValueOf(f.Index(i))) {
			if count > 0 {
				f.Remove(i-count, i)
				i -= count
			}
			count = 0
		} else {
			count++
		}
	}
	if count > 0 {
		f.Remove(i-count, i)
	}
}