package convert

import (
	"bytes"
	"fmt"
	"sort"

	"github.com/zclconf/go-cty/cty"
)

// MismatchMessage is a helper to return an English-language description of
// the differences between got and want, phrased as a reason why got does
// not conform to want.
//
// This function does not itself attempt conversion, and so it should generally
// be used only after a conversion has failed, to report the conversion failure
// to an English-speaking user. The result will be confusing got is actually
// conforming to or convertable to want.
//
// The shorthand helper function Convert uses this function internally to
// produce its error messages, so callers of that function do not need to
// also use MismatchMessage.
//
// This function is similar to Type.TestConformance, but it is tailored to
// describing conversion failures and so the messages it generates relate
// specifically to the conversion rules implemented in this package.
func MismatchMessage(got, want cty.Type) string {
	switch {

	case got.IsObjectType() && want.IsObjectType():
		// If both types are object types then we may be able to say something
		// about their respective attributes.
		return mismatchMessageObjects(got, want)

	case got.IsTupleType() && want.IsListType() && want.ElementType() == cty.DynamicPseudoType:
		// If conversion from tuple to list failed then it's because we couldn't
		// find a common type to convert all of the tuple elements to.
		return "all list elements must have the same type"

	case got.IsTupleType() && want.IsSetType() && want.ElementType() == cty.DynamicPseudoType:
		// If conversion from tuple to set failed then it's because we couldn't
		// find a common type to convert all of the tuple elements to.
		return "all set elements must have the same type"

	case got.IsObjectType() && want.IsMapType() && want.ElementType() == cty.DynamicPseudoType:
		// If conversion from object to map failed then it's because we couldn't
		// find a common type to convert all of the object attributes to.
		return "all map elements must have the same type"

	case (got.IsTupleType() || got.IsObjectType()) && want.IsCollectionType():
		return mismatchMessageCollectionsFromStructural(got, want)

	case got.IsCollectionType() && want.IsCollectionType():
		return mismatchMessageCollectionsFromCollections(got, want)

	case !typesAreLikelyToCauseConfusion(got, want):
		return fmt.Sprintf("%s required, but have %s", want.FriendlyName(), got.FriendlyName())

	default:
		// If we have nothing better to say, we'll just state what was required.
		return want.FriendlyNameForConstraint() + " required"
	}
}

func mismatchMessageObjects(got, want cty.Type) string {
	// Per our conversion rules, "got" is allowed to be a superset of "want",
	// and so we'll produce error messages here under that assumption.
	gotAtys := got.AttributeTypes()
	wantAtys := want.AttributeTypes()

	// If we find missing attributes then we'll report those in preference,
	// but if not then we will report a maximum of one non-conforming
	// attribute, just to keep our messages relatively terse.
	// We'll also prefer to report a recursive type error from an _unsafe_
	// conversion over a safe one, because these are subjectively more
	// "serious".
	var missingAttrs []string
	var unsafeMismatchAttr string
	var safeMismatchAttr string

	for name, wantAty := range wantAtys {
		gotAty, exists := gotAtys[name]
		if !exists {
			if !want.AttributeOptional(name) {
				missingAttrs = append(missingAttrs, name)
			}
			continue
		}

		if gotAty.Equals(wantAty) {
			continue // exact match, so no problem
		}

		// We'll now try to convert these attributes in isolation and
		// see if we have a nested conversion error to report.
		// We'll try an unsafe conversion first, and then fall back on
		// safe if unsafe is possible.

		// If we already have an unsafe mismatch attr error then we won't bother
		// hunting for another one.
		if unsafeMismatchAttr != "" {
			continue
		}
		if conv := GetConversionUnsafe(gotAty, wantAty); conv == nil {
			unsafeMismatchAttr = fmt.Sprintf("attribute %q: %s", name, MismatchMessage(gotAty, wantAty))
		}

		// If we already have a safe mismatch attr error then we won't bother
		// hunting for another one.
		if safeMismatchAttr != "" {
			continue
		}
		if conv := GetConversion(gotAty, wantAty); conv == nil {
			safeMismatchAttr = fmt.Sprintf("attribute %q: %s", name, MismatchMessage(gotAty, wantAty))
		}
	}

	// We should now have collected at least one problem. If we have more than
	// one then we'll use our preference order to decide what is most important
	// to report.
	switch {

	case len(missingAttrs) != 0:
		sort.Strings(missingAttrs)
		switch len(missingAttrs) {
		case 1:
			return fmt.Sprintf("attribute %q is required", missingAttrs[0])
		case 2:
			return fmt.Sprintf("attributes %q and %q are required", missingAttrs[0], missingAttrs[1])
		default:
			sort.Strings(missingAttrs)
			var buf bytes.Buffer
			for _, name := range missingAttrs[:len(missingAttrs)-1] {
				fmt.Fprintf(&buf, "%q, ", name)
			}
			fmt.Fprintf(&buf, "and %q", missingAttrs[len(missingAttrs)-1])
			return fmt.Sprintf("attributes %s are required", buf.Bytes())
		}

	case unsafeMismatchAttr != "":
		return unsafeMismatchAttr

	case safeMismatchAttr != "":
		return safeMismatchAttr

	default:
		// We should never get here, but if we do then we'll return
		// just a generic message.
		return "incorrect object attributes"
	}
}

func mismatchMessageCollectionsFromStructural(got, want cty.Type) string {
	// First some straightforward cases where the kind is just altogether wrong.
	switch {
	case want.IsListType() && !got.IsTupleType():
		return want.FriendlyNameForConstraint() + " required"
	case want.IsSetType() && !got.IsTupleType():
		return want.FriendlyNameForConstraint() + " required"
	case want.IsMapType() && !got.IsObjectType():
		return want.FriendlyNameForConstraint() + " required"
	}

	// If the kinds are matched well enough then we'll move on to checking
	// individual elements.
	wantEty := want.ElementType()
	switch {
	case got.IsTupleType():
		for i, gotEty := range got.TupleElementTypes() {
			if gotEty.Equals(wantEty) {
				continue // exact match, so no problem
			}
			if conv := getConversion(gotEty, wantEty, true); conv != nil {
				continue // conversion is available, so no problem
			}
			return fmt.Sprintf("element %d: %s", i, MismatchMessage(gotEty, wantEty))
		}

		// If we get down here then something weird is going on but we'll
		// return a reasonable fallback message anyway.
		return fmt.Sprintf("all elements must be %s", wantEty.FriendlyNameForConstraint())

	case got.IsObjectType():
		for name, gotAty := range got.AttributeTypes() {
			if gotAty.Equals(wantEty) {
				continue // exact match, so no problem
			}
			if conv := getConversion(gotAty, wantEty, true); conv != nil {
				continue // conversion is available, so no problem
			}
			return fmt.Sprintf("element %q: %s", name, MismatchMessage(gotAty, wantEty))
		}

		// If we get down here then something weird is going on but we'll
		// return a reasonable fallback message anyway.
		return fmt.Sprintf("all elements must be %s", wantEty.FriendlyNameForConstraint())

	default:
		// Should not be possible to get here since we only call this function
		// with got as structural types, but...
		return want.FriendlyNameForConstraint() + " required"
	}
}

func mismatchMessageCollectionsFromCollections(got, want cty.Type) string {
	// First some straightforward cases where the kind is just altogether wrong.
	switch {
	case want.IsListType() && !(got.IsListType() || got.IsSetType()):
		return want.FriendlyNameForConstraint() + " required"
	case want.IsSetType() && !(got.IsListType() || got.IsSetType()):
		return want.FriendlyNameForConstraint() + " required"
	case want.IsMapType() && !got.IsMapType():
		return want.FriendlyNameForConstraint() + " required"
	}

	// If the kinds are matched well enough then we'll check the element types.
	gotEty := got.ElementType()
	wantEty := want.ElementType()
	noun := "element type"
	switch {
	case want.IsListType():
		noun = "list element type"
	case want.IsSetType():
		noun = "set element type"
	case want.IsMapType():
		noun = "map element type"
	}
	return fmt.Sprintf("incorrect %s: %s", noun, MismatchMessage(gotEty, wantEty))
}

func typesAreLikelyToCauseConfusion(got, want cty.Type) bool {
	// NOTE: This function is intended to be used as the penultemate test
	// in MismatchMessage, and so it intentionally does not address
	// combinations that are already addressed by the more specific
	// earlier tests.
	if gotP, wantP := got.IsPrimitiveType(), want.IsPrimitiveType(); gotP != wantP {
		// There's never any situation where a primitive type could successfully
		// convert to a non-primitive one, so this is very unlikely to be
		// intended as an automatic type conversion. This case is doing most
		// of the useful work of this function, allowing us to report such
		// things as "string required, but got tuple".
		return false
	}
	if want == cty.String {
		// All of the primitive types can always successfully convert to
		// string, regardless of value.
		return false
	}
	if (want == cty.Bool && got == cty.Number) || (got == cty.Bool && want == cty.Number) {
		// There are no automatic conversions between bool and number, so
		// describing both directions of this is unlikely to cause confusion.
		return false
	}
	if got.IsCollectionType() && want.IsCollectionType() && !got.ElementType().HasDynamicTypes() && want.ElementType().HasDynamicTypes() {
		// Describing mismatches of collection element types is helpful
		// when we have specific types to report, but confusing when
		// there are any inexact types involved because readers tend to
		// then think the inexactness of the element type is the problem,
		// when it's far more likely to be the collection type kind that's
		// causing the problem.
		return false
	}
	if got.IsTupleType() && want.IsObjectType() || got.IsObjectType() && got.IsTupleType() {
		// The structural type kinds get described just by their bare names,
		// "tuple" or "object", and so they are not confusing when they
		// differ. They _are_ potentially confusing when we have matching kinds
		// but different element types within, but thankfully that's most
		// often handled by one of the earlier cases in MismatchMessage.
		return false
	}
	// We are intentionally a little more vague for string-to-number and
	// string-to-bool conversions because the rules for those are a little
	// too subtle to be described purely as "need X but want Y", and historical
	// experience shows that describing it that way causes folks to focus
	// on the wrong problem (their value is wrong, not their type).

	// If we find that we're getting here in additional specific cases where
	// it would be more helpful than confusing to report both types then
	// we'll add additional rules here in a future version.
	return true
}