/*
Package validator implements value validations for structs and individual fields based on tags. It can also handle Cross Field validation and even Cross Field Cross Struct validation for nested structs.

Built In Validator

	myValidator = validator.NewValidator("validate", validator.BakedInValidators)

	errs := myValidator.ValidateStruct(//your struct)
	valErr := myValidator.ValidateFieldByTag(field, "omitempty,min=1,max=10")

A simple example usage:

	type UserDetail {
		Details string `validate:"-"`
	}

	type User struct {
		Name         string     `validate:"required,max=60"`
		PreferedName string     `validate:"omitempty,max=60"`
		Sub          UserDetail
	}

	user := &User {
		Name: "",
	}

	// errs will contain a hierarchical list of errors
	// using the StructValidationErrors struct
	// or nil if no errors exist
	errs := myValidator.ValidateStruct(user)

	// in this case 1 error Name is required
	errs.Struct will be "User"
	errs.StructErrors will be empty <-- fields that were structs
	errs.Errors will have 1 error of type FieldValidationError

	NOTE: Anonymous Structs - they don't have names so expect the Struct name
	within StructValidationErrors to be blank.

Error Handling

The error can be used like so

	fieldErr, _ := errs["Name"]
	fieldErr.Field    // "Name"
	fieldErr.ErrorTag // "required"

Both StructValidationErrors and FieldValidationError implement the Error interface but it's
intended use is for development + debugging, not a production error message.

	fieldErr.Error() // Field validation for "Name" failed on the "required" tag
	errs.Error()
	// Struct: User
	// Field validation for "Name" failed on the "required" tag

Why not a better error message? because this library intends for you to handle your own error messages

Why should I handle my own errors? Many reasons, for me building an internationalized application
I needed to know the field and what validation failed so that I could provide an error in the users specific language.

	if fieldErr.Field == "Name" {
		switch fieldErr.ErrorTag
		case "required":
			return "Translated string based on field + error"
		default:
		return "Translated string based on field"
	}

The hierarchical error structure is hard to work with sometimes.. Agreed Flatten function to the rescue!
Flatten will return a map of FieldValidationError's but the field name will be namespaced.

	// if UserDetail Details field failed validation
	Field will be "Sub.Details"

	// for Name
	Field will be "Name"

Custom Functions

Custom functions can be added

	//Structure
	func customFunc(top interface{}, current interface{}, field interface{}, param string) bool {

		if whatever {
			return false
		}

		return true
	}

	myValidator.AddFunction("custom tag name", customFunc)
	// NOTES: using the same tag name as an existing function
	//        will overwrite the existing one

Cross Field Validation

Cross Field Validation can be implemented, for example Start & End Date range validation

	// NOTE: when calling myValidator.validateStruct(val) val will be the top level struct passed
	//       into the function
	//       when calling myValidator.ValidateFieldByTagAndValue(val, field, tag) val will be
	//       whatever you pass, struct, field...
	//       when calling myValidator.ValidateFieldByTag(field, tag) val will be nil
	//
	// Because of the specific requirements and field names within each persons project that
	// uses this library it is likely that custom functions will need to be created for your
	// Cross Field Validation needs, however there are some build in Generic Cross Field validations,
	// see Baked In Validators and Tags below

	func isDateRangeValid(val interface{}, field interface{}, param string) bool {

		myStruct := val.(myStructType)

		if myStruct.Start.After(field.(time.Time)) {
			return false
		}

		return true
	}

Multiple Validators

Multiple validators on a field will process in the order defined

	type Test struct {
		Field `validate:"max=10,min=1"`
	}

	// max will be checked then min

Bad Validator definitions are not handled by the library

	type Test struct {
		Field `validate:"min=10,max=0"`
	}

	// this definition of min max will never validate

Baked In Validators and Tags

NOTE: Baked In Cross field validation only compares fields on the same struct,
if cross field + cross struct validation is needed your own custom validator
should be implemented.

Here is a list of the current built in validators:

	-
		Tells the validation to skip this struct field; this is particularily
		handy in ignoring embedded structs from being validated. (Usage: -)

	|
		This is the 'or' operator allowing multiple validators to be used and
		accepted. (Usage: rbg|rgba) <-- this would allow either rgb or rgba
		colors to be accepted. This can also be combined with 'and' for example
		( Usage: omitempty,rgb|rgba)

	structonly
		When a field that is a nest struct in encountered and contains this flag
		any validation on the nested struct such as "required" will be run, but
		none of the nested struct fields will be validated. This is usefull if
		inside of you program you know the struct will be valid, but need to
		verify it has been assigned.

	omitempty
		Allows conitional validation, for example if a field is not set with
		a value (Determined by the required validator) then other validation
		such as min or max won't run, but if a value is set validation will run.
		(Usage: omitempty)

	required
		This validates that the value is not the data types default value.
		For numbers ensures value is not zero. For strings ensures value is
		not "". For slices, arrays, and maps, ensures the length is not zero.
		(Usage: required)

	len
		For numbers, max will ensure that the value is
		equal to the parameter given. For strings, it checks that
		the string length is exactly that number of characters. For slices,
		arrays, and maps, validates the number of items. (Usage: len=10)

	max
		For numbers, max will ensure that the value is
		less than or equal to the parameter given. For strings, it checks
		that the string length is at most that number of characters. For
		slices, arrays, and maps, validates the number of items. (Usage: max=10)

	min
		For numbers, min will ensure that the value is
		greater or equal to the parameter given. For strings, it checks that
		the string length is at least that number of characters. For slices,
		arrays, and maps, validates the number of items. (Usage: min=10)

	gt
		For numbers, this will ensure that the value is greater than the
		parameter given. For strings, it checks that the string length
		is greater than that number of characters. For slices, arrays
		and maps it validates the number of items. (Usage: gt=10)
		For time.Time ensures the time value is greater than time.Now.UTC()
		(Usage: gt)

	gte
		Same as 'min' above. Kept both to make terminology with 'len' easier
		(Usage: gte=10)
		For time.Time ensures the time value is greater than or equal to time.Now.UTC()
		(Usage: gte)

	lt
		For numbers, this will ensure that the value is
		less than the parameter given. For strings, it checks
		that the string length is less than that number of characters.
		For slices, arrays, and maps it validates the number of items.
		(Usage: lt=10)
		For time.Time ensures the time value is less than time.Now.UTC()
		(Usage: lt)

	lte
		Same as 'max' above. Kept both to make terminology with 'len' easier
		(Usage: lte=10)
		For time.Time ensures the time value is less than or equal to time.Now.UTC()
		(Usage: lte)

	gtfield
		Only valid for Numbers and time.Time types, this will validate the field value
		against another fields value either within a struct or passed in field.
		usage examples are for validation of a Start and End date:
		Validation on End field using ValidateByStruct Usage(gtfield=Start)
		Validating by field ValidateFieldByTagAndValue(start, end, "gtfield")

	gtefield
		Only valid for Numbers and time.Time types, this will validate the field value
		against another fields value either within a struct or passed in field.
		usage examples are for validation of a Start and End date:
		Validation on End field using ValidateByStruct Usage(gtefield=Start)
		Validating by field ValidateFieldByTagAndValue(start, end, "gtefield")

	ltfield
		Only valid for Numbers and time.Time types, this will validate the field value
		against another fields value either within a struct or passed in field.
		usage examples are for validation of a Start and End date:
		Validation on End field using ValidateByStruct Usage(ltfield=Start)
		Validating by field ValidateFieldByTagAndValue(start, end, "ltfield")

	ltefield
		Only valid for Numbers and time.Time types, this will validate the field value
		against another fields value either within a struct or passed in field.
		usage examples are for validation of a Start and End date:
		Validation on End field using ValidateByStruct Usage(ltefield=Start)
		Validating by field ValidateFieldByTagAndValue(start, end, "ltefield")

	alpha
		This validates that a strings value contains alpha characters only
		(Usage: alpha)

	alphanum
		This validates that a strings value contains alphanumeric characters only
		(Usage: alphanum)

	numeric
		This validates that a strings value contains a basic numeric value.
		basic excludes exponents etc...
		(Usage: numeric)

	hexadecimal
		This validates that a strings value contains a valid hexadecimal.
		(Usage: hexadecimal)

	hexcolor
		This validates that a strings value contains a valid hex color including
		hashtag (#)
		(Usage: hexcolor)

	rgb
		This validates that a strings value contains a valid rgb color
		(Usage: rgb)

	rgba
		This validates that a strings value contains a valid rgba color
		(Usage: rgba)

	hsl
		This validates that a strings value contains a valid hsl color
		(Usage: hsl)

	hsla
		This validates that a strings value contains a valid hsla color
		(Usage: hsla)

	email
		This validates that a strings value contains a valid email
		This may not conform to all possibilities of any rfc standard, but neither
		does any email provider accept all posibilities...
		(Usage: email)
	url
		This validates that a strings value contains a valid url
		This will accept any url the golang request uri accepts but must contain
		a schema for example http:// or rtmp://
		(Usage: url)
	uri
		This validates that a strings value contains a valid uri
		This will accept any uri the golang request uri accepts (Usage: uri)

Validator notes:

	regex
		a regex validator won't be added because commas and = signs can be part of
		a regex which conflict with the validation definitions, although workarounds
		can be made, they take away from using pure regex's. Furthermore it's quick
		and dirty but the regex's become harder to maintain and are not reusable, so
		it's as much a programming philosiphy as anything.

		In place of this new validator functions should be created; a regex can be
		used within the validator function and even be precompiled for better efficiency
		within regexes.go.

		And the best reason, you can sumit a pull request and we can keep on adding to the
		validation library of this package!

Panics

This package panics when bad input is provided, this is by design, bad code like that should not make it to production.

	type Test struct {
		TestField string `validate:"nonexistantfunction=1"`
	}

	t := &Test{
		TestField: "Test"
	}

	myValidator.ValidateStruct(t) // this will panic
*/
package validator