package runtime

// This file implements Go interfaces.
//
// Interfaces are represented as a pair of {typecode, value}, where value can be
// anything (including non-pointers).
//
// Signatures itself are not matched on strings, but on uniqued numbers that
// contain the name and the signature of the function (to save space), think of
// signatures as interned strings at compile time.
//
// The typecode is a small number unique for the Go type. All typecodes <
// firstTypeWithMethods do not have any methods and typecodes >=
// firstTypeWithMethods all have at least one method. This means that
// methodSetRanges does not need to contain types without methods and is thus
// indexed starting at a typecode with number firstTypeWithMethods.
//
// To further conserve some space, the methodSetRange (as the name indicates)
// doesn't contain a list of methods and function pointers directly, but instead
// just indexes into methodSetSignatures and methodSetFunctions which contains
// the mapping from uniqued signature to function pointer.

type _interface struct {
	typecode uint16
	value    *uint8
}

// This struct indicates the range of methods in the methodSetSignatures and
// methodSetFunctions arrays that belong to this named type.
type methodSetRange struct {
	index  uint16 // start index into interfaceSignatures and interfaceFunctions
	length uint16 // number of methods
}

// Global constants that will be set by the compiler. The arrays are of size 0,
// which is a dummy value, but will be bigger after the compiler has filled them
// in.
var (
	firstTypeWithMethods uint16            // the lowest typecode that has at least one method
	methodSetRanges      [0]methodSetRange // indices into methodSetSignatures and methodSetFunctions
	methodSetSignatures  [0]uint16         // uniqued method ID
	methodSetFunctions   [0]*uint8         // function pointer of method
	interfaceIndex       [0]uint16         // mapping from interface ID to an index in interfaceMethods
	interfaceLengths     [0]uint8          // mapping from interface ID to the number of methods it has
	interfaceMethods     [0]uint16         // the method an interface implements (list of method IDs)
)

// Get the function pointer for the method on the interface.
// This is a compiler intrinsic.
//go:nobounds
func interfaceMethod(itf _interface, method uint16) *uint8 {
	// This function doesn't do bounds checking as the supplied method must be
	// in the list of signatures. The compiler will only emit
	// runtime.interfaceMethod calls when the method actually exists on this
	// interface (proven by the typechecker).
	i := methodSetRanges[itf.typecode-firstTypeWithMethods].index
	for {
		if methodSetSignatures[i] == method {
			return methodSetFunctions[i]
		}
		i++
	}
}

// Return true iff both interfaces are equal.
func interfaceEqual(x, y _interface) bool {
	if x.typecode != y.typecode {
		// Different dynamic type so always unequal.
		return false
	}
	if x.typecode == 0 {
		// Both interfaces are nil, so they are equal.
		return true
	}
	// TODO: depends on reflection.
	panic("unimplemented: interface equality")
}

// Return true iff the type implements all methods needed by the interface. This
// means the type satisfies the interface.
// This is a compiler intrinsic.
//go:nobounds
func interfaceImplements(typecode, interfaceNum uint16) bool {
	// method set indices of the interface
	itfIndex := interfaceIndex[interfaceNum]
	itfIndexEnd := itfIndex + uint16(interfaceLengths[interfaceNum])

	if itfIndex == itfIndexEnd {
		// This interface has no methods, so it satisfies all types.
		// TODO: this should be figured out at compile time (as it is known at
		// compile time), so that this check is unnecessary at runtime.
		return true
	}

	if typecode < firstTypeWithMethods {
		// Type has no methods while the interface has (checked above), so this
		// type does not satisfy this interface.
		return false
	}

	// method set indices of the concrete type
	methodSet := methodSetRanges[typecode-firstTypeWithMethods]
	methodIndex := methodSet.index
	methodIndexEnd := methodSet.index + methodSet.length

	// Iterate over all methods of the interface:
	for itfIndex < itfIndexEnd {
		methodId := interfaceMethods[itfIndex]
		if methodIndex >= methodIndexEnd {
			// Reached the end of the list of methods, so interface doesn't
			// implement this type.
			return false
		}
		if methodId == methodSetSignatures[methodIndex] {
			// Found a matching method, continue to the next method.
			itfIndex++
			methodIndex++
			continue
		} else if methodId > methodSetSignatures[methodIndex] {
			// The method didn't match, but method ID of the concrete type was
			// lower than that of the interface, so probably it has a method the
			// interface doesn't implement.
			// Move on to the next method of the concrete type.
			methodIndex++
			continue
		} else {
			// The concrete type is missing a method. This means the type assert
			// fails.
			return false
		}
	}

	// Found a method for each expected method in the interface. This type
	// assert is successful.
	return true
}

func interfaceTypeAssert(ok bool) {
	if !ok {
		runtimePanic("type assert failed")
	}
}