fp-go/v2/endomorphism/endomorphism_test.go

// Copyright (c) 2023 - 2025 IBM Corp.
// All rights reserved.
//
// 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 endomorphism

import (
	"testing"

	M "github.com/IBM/fp-go/v2/monoid"
	S "github.com/IBM/fp-go/v2/semigroup"
	"github.com/stretchr/testify/assert"
)

// Test helper functions
func double(x int) int {
	return x * 2
}

func increment(x int) int {
	return x + 1
}

func square(x int) int {
	return x * x
}

func negate(x int) int {
	return -x
}

// TestCurry2 tests the Curry2 function
func TestCurry2(t *testing.T) {
	add := func(x, y int) int {
		return x + y
	}

	curriedAdd := Curry2(add)
	addFive := curriedAdd(5)

	result := addFive(10)
	assert.Equal(t, 15, result, "Curry2 should curry binary function correctly")

	// Test with different values
	addTen := curriedAdd(10)
	assert.Equal(t, 25, addTen(15), "Curry2 should work with different values")
}

// TestCurry3 tests the Curry3 function
func TestCurry3(t *testing.T) {
	combine := func(x, y, z int) int {
		return x + y + z
	}

	curriedCombine := Curry3(combine)
	addTen := curriedCombine(5)(5)

	result := addTen(20)
	assert.Equal(t, 30, result, "Curry3 should curry ternary function correctly")

	// Test with different values
	addFifteen := curriedCombine(5)(10)
	assert.Equal(t, 35, addFifteen(20), "Curry3 should work with different values")
}

// TestMonadAp tests the MonadAp function
func TestMonadAp(t *testing.T) {
	result := MonadAp(double, 5)
	assert.Equal(t, 10, result, "MonadAp should apply endomorphism to value")

	result2 := MonadAp(increment, 10)
	assert.Equal(t, 11, result2, "MonadAp should work with different endomorphisms")

	result3 := MonadAp(square, 4)
	assert.Equal(t, 16, result3, "MonadAp should work with square function")
}

// TestAp tests the Ap function
func TestAp(t *testing.T) {
	applyFive := Ap(5)

	result := applyFive(double)
	assert.Equal(t, 10, result, "Ap should apply value to endomorphism")

	result2 := applyFive(increment)
	assert.Equal(t, 6, result2, "Ap should work with different endomorphisms")

	applyTen := Ap(10)
	result3 := applyTen(square)
	assert.Equal(t, 100, result3, "Ap should work with different values")
}

// TestMonadCompose tests the MonadCompose function
func TestMonadCompose(t *testing.T) {
	// Test basic composition: RIGHT-TO-LEFT execution
	// MonadCompose(double, increment) means: increment first, then double
	composed := MonadCompose(double, increment)
	result := composed(5)
	assert.Equal(t, 12, result, "MonadCompose should execute right-to-left: (5 + 1) * 2 = 12")

	// Test composition order: RIGHT-TO-LEFT execution
	// MonadCompose(increment, double) means: double first, then increment
	composed2 := MonadCompose(increment, double)
	result2 := composed2(5)
	assert.Equal(t, 11, result2, "MonadCompose should execute right-to-left: (5 * 2) + 1 = 11")

	// Test with three compositions: RIGHT-TO-LEFT execution
	// MonadCompose(MonadCompose(double, increment), square) means: square, then increment, then double
	complex := MonadCompose(MonadCompose(double, increment), square)
	result3 := complex(5)
	// 5 -> square -> 25 -> increment -> 26 -> double -> 52
	assert.Equal(t, 52, result3, "MonadCompose should work with nested compositions: square(5)=25, +1=26, *2=52")
}

// TestMonadChain tests the MonadChain function
func TestMonadChain(t *testing.T) {
	// MonadChain executes LEFT-TO-RIGHT (first arg first, second arg second)
	chained := MonadChain(double, increment)
	result := chained(5)
	assert.Equal(t, 11, result, "MonadChain should execute left-to-right: (5 * 2) + 1 = 11")

	chained2 := MonadChain(increment, double)
	result2 := chained2(5)
	assert.Equal(t, 12, result2, "MonadChain should execute left-to-right: (5 + 1) * 2 = 12")

	// Test with negative values
	chained3 := MonadChain(negate, increment)
	result3 := chained3(5)
	assert.Equal(t, -4, result3, "MonadChain should execute left-to-right: -(5) + 1 = -4")
}

// TestChain tests the Chain function
func TestChain(t *testing.T) {
	// Chain(f) returns a function that applies its argument first, then f
	chainWithIncrement := Chain(increment)

	// chainWithIncrement(double) means: double first, then increment
	chained := chainWithIncrement(double)
	result := chained(5)
	assert.Equal(t, 11, result, "Chain should execute left-to-right: (5 * 2) + 1 = 11")

	chainWithDouble := Chain(double)
	// chainWithDouble(increment) means: increment first, then double
	chained2 := chainWithDouble(increment)
	result2 := chained2(5)
	assert.Equal(t, 12, result2, "Chain should execute left-to-right: (5 + 1) * 2 = 12")

	// Test chaining with square
	chainWithSquare := Chain(square)
	// chainWithSquare(double) means: double first, then square
	chained3 := chainWithSquare(double)
	result3 := chained3(3)
	assert.Equal(t, 36, result3, "Chain should execute left-to-right: (3 * 2) ^ 2 = 36")
}

// TestCompose tests the curried Compose function
func TestCompose(t *testing.T) {
	// Compose(g) returns a function that applies g first, then its argument
	composeWithIncrement := Compose(increment)

	// composeWithIncrement(double) means: increment first, then double
	composed := composeWithIncrement(double)
	result := composed(5)
	assert.Equal(t, 12, result, "Compose should execute right-to-left: (5 + 1) * 2 = 12")

	composeWithDouble := Compose(double)
	// composeWithDouble(increment) means: double first, then increment
	composed2 := composeWithDouble(increment)
	result2 := composed2(5)
	assert.Equal(t, 11, result2, "Compose should execute right-to-left: (5 * 2) + 1 = 11")

	// Test composing with square
	composeWithSquare := Compose(square)
	// composeWithSquare(double) means: square first, then double
	composed3 := composeWithSquare(double)
	result3 := composed3(3)
	assert.Equal(t, 18, result3, "Compose should execute right-to-left: (3 ^ 2) * 2 = 18")
}

// TestMonadComposeVsCompose demonstrates the relationship between MonadCompose and Compose
func TestMonadComposeVsCompose(t *testing.T) {
	double := func(x int) int { return x * 2 }
	increment := func(x int) int { return x + 1 }

	// MonadCompose takes both functions at once
	monadComposed := MonadCompose(double, increment)
	result1 := monadComposed(5) // (5 + 1) * 2 = 12

	// Compose is the curried version - takes one function, returns a function
	curriedCompose := Compose(increment)
	composed := curriedCompose(double)
	result2 := composed(5) // (5 + 1) * 2 = 12

	assert.Equal(t, result1, result2, "MonadCompose and Compose should produce the same result")
	assert.Equal(t, 12, result1, "Both should execute right-to-left: (5 + 1) * 2 = 12")

	// Demonstrate that Compose(g)(f) is equivalent to MonadCompose(f, g)
	assert.Equal(t, MonadCompose(double, increment)(5), Compose(increment)(double)(5),
		"Compose(g)(f) should equal MonadCompose(f, g)")
}

// TestOf tests the Of function
func TestOf(t *testing.T) {
	endo := Of(double)
	result := endo(5)
	assert.Equal(t, 10, result, "Of should convert function to endomorphism")

	endo2 := Of(increment)
	result2 := endo2(10)
	assert.Equal(t, 11, result2, "Of should work with different functions")
}

// TestWrap tests the Wrap function (deprecated)
func TestWrap(t *testing.T) {
	endo := Wrap(double)
	result := endo(5)
	assert.Equal(t, 10, result, "Wrap should convert function to endomorphism")
}

// TestUnwrap tests the Unwrap function (deprecated)
func TestUnwrap(t *testing.T) {
	endo := Of(double)
	unwrapped := Unwrap[func(int) int](endo)
	result := unwrapped(5)
	assert.Equal(t, 10, result, "Unwrap should convert endomorphism to function")
}

// TestIdentity tests the Identity function
func TestIdentity(t *testing.T) {
	id := Identity[int]()

	// Identity should return input unchanged
	assert.Equal(t, 42, id(42), "Identity should return input unchanged")
	assert.Equal(t, 0, id(0), "Identity should work with zero")
	assert.Equal(t, -10, id(-10), "Identity should work with negative values")

	// Identity should be neutral for composition (RIGHT-TO-LEFT)
	// Compose(id, double) means: double first, then id
	composed1 := MonadCompose(id, double)
	assert.Equal(t, 10, composed1(5), "Identity should be left neutral: double(5) = 10")

	// Compose(double, id) means: id first, then double
	composed2 := MonadCompose(double, id)
	assert.Equal(t, 10, composed2(5), "Identity should be right neutral: id(5) then double = 10")

	// Test with strings
	idStr := Identity[string]()
	assert.Equal(t, "hello", idStr("hello"), "Identity should work with strings")
}

// TestSemigroup tests the Semigroup function
func TestSemigroup(t *testing.T) {
	sg := Semigroup[int]()

	// Test basic concat (RIGHT-TO-LEFT execution via Compose)
	// Concat(double, increment) means: increment first, then double
	combined := sg.Concat(double, increment)
	result := combined(5)
	assert.Equal(t, 12, result, "Semigroup concat should execute right-to-left: (5 + 1) * 2 = 12")

	// Test associativity: (f . g) . h = f . (g . h)
	f := double
	g := increment
	h := square

	left := sg.Concat(sg.Concat(f, g), h)
	right := sg.Concat(f, sg.Concat(g, h))

	testValue := 3
	assert.Equal(t, left(testValue), right(testValue), "Semigroup should be associative")

	// Test with ConcatAll from semigroup package (RIGHT-TO-LEFT)
	// ConcatAll(double)(increment, square) means: square, then increment, then double
	combined2 := S.ConcatAll(sg)(double)([]Endomorphism[int]{increment, square})
	result2 := combined2(5)
	// 5 -> square -> 25 -> increment -> 26 -> double -> 52
	assert.Equal(t, 52, result2, "Semigroup ConcatAll should execute right-to-left: square(5)=25, +1=26, *2=52")
}

// TestMonoid tests the Monoid function
func TestMonoid(t *testing.T) {
	monoid := Monoid[int]()

	// Test that empty is identity
	empty := monoid.Empty()
	assert.Equal(t, 42, empty(42), "Monoid empty should be identity")

	// Test right identity: x . empty = x (RIGHT-TO-LEFT: empty first, then x)
	// Concat(double, empty) means: empty first, then double
	rightIdentity := monoid.Concat(double, empty)
	assert.Equal(t, 10, rightIdentity(5), "Monoid should satisfy right identity: empty(5) then double = 10")

	// Test left identity: empty . x = x (RIGHT-TO-LEFT: x first, then empty)
	// Concat(empty, double) means: double first, then empty
	leftIdentity := monoid.Concat(empty, double)
	assert.Equal(t, 10, leftIdentity(5), "Monoid should satisfy left identity: double(5) then empty = 10")

	// Test ConcatAll with multiple endomorphisms (RIGHT-TO-LEFT execution)
	combined := M.ConcatAll(monoid)([]Endomorphism[int]{double, increment, square})
	result := combined(5)
	// RIGHT-TO-LEFT: square(5) = 25, increment(25) = 26, double(26) = 52
	assert.Equal(t, 52, result, "Monoid ConcatAll should execute right-to-left: square(5)=25, +1=26, *2=52")

	// Test ConcatAll with empty list should return identity
	emptyResult := M.ConcatAll(monoid)([]Endomorphism[int]{})
	assert.Equal(t, 42, emptyResult(42), "ConcatAll with no args should return identity")
}

// TestMonoidLaws tests that the Monoid satisfies monoid laws
func TestMonoidLaws(t *testing.T) {
	monoid := Monoid[int]()
	empty := monoid.Empty()

	testCases := []struct {
		name string
		f    Endomorphism[int]
		g    Endomorphism[int]
		h    Endomorphism[int]
	}{
		{"basic", double, increment, square},
		{"with negate", negate, double, increment},
		{"with identity", Identity[int](), double, increment},
	}

	for _, tc := range testCases {
		t.Run(tc.name, func(t *testing.T) {
			testValue := 5

			// Right identity: x . empty = x
			rightId := monoid.Concat(tc.f, empty)
			assert.Equal(t, tc.f(testValue), rightId(testValue), "Right identity law")

			// Left identity: empty . x = x
			leftId := monoid.Concat(empty, tc.f)
			assert.Equal(t, tc.f(testValue), leftId(testValue), "Left identity law")

			// Associativity: (f . g) . h = f . (g . h)
			left := monoid.Concat(monoid.Concat(tc.f, tc.g), tc.h)
			right := monoid.Concat(tc.f, monoid.Concat(tc.g, tc.h))
			assert.Equal(t, left(testValue), right(testValue), "Associativity law")
		})
	}
}

// TestEndomorphismWithDifferentTypes tests endomorphisms with different types
func TestEndomorphismWithDifferentTypes(t *testing.T) {
	// Test with strings (RIGHT-TO-LEFT execution)
	addExclamation := func(s string) string {
		return s + "!"
	}
	addPrefix := func(s string) string {
		return "Hello, " + s
	}

	// Compose(addExclamation, addPrefix) means: addPrefix first, then addExclamation
	strComposed := MonadCompose(addExclamation, addPrefix)
	result := strComposed("World")
	assert.Equal(t, "Hello, World!", result, "Compose should execute right-to-left with strings")

	// Test with float64 (RIGHT-TO-LEFT execution)
	doubleFloat := func(x float64) float64 {
		return x * 2.0
	}
	addOne := func(x float64) float64 {
		return x + 1.0
	}

	// Compose(doubleFloat, addOne) means: addOne first, then doubleFloat
	floatComposed := MonadCompose(doubleFloat, addOne)
	resultFloat := floatComposed(5.5)
	// 5.5 + 1.0 = 6.5, 6.5 * 2.0 = 13.0
	assert.Equal(t, 13.0, resultFloat, "Compose should execute right-to-left: (5.5 + 1.0) * 2.0 = 13.0")
}

// TestComplexCompositions tests more complex composition scenarios
func TestComplexCompositions(t *testing.T) {
	// Create a pipeline of transformations (RIGHT-TO-LEFT execution)
	// Innermost Compose is evaluated first in the composition chain
	pipeline := MonadCompose(
		MonadCompose(
			MonadCompose(double, increment),
			square,
		),
		negate,
	)

	// RIGHT-TO-LEFT: negate(5) = -5, square(-5) = 25, increment(25) = 26, double(26) = 52
	result := pipeline(5)
	assert.Equal(t, 52, result, "Complex composition should execute right-to-left")

	// Test using monoid to build the same pipeline (RIGHT-TO-LEFT)
	monoid := Monoid[int]()
	pipelineMonoid := M.ConcatAll(monoid)([]Endomorphism[int]{double, increment, square, negate})
	resultMonoid := pipelineMonoid(5)
	// RIGHT-TO-LEFT: negate(5) = -5, square(-5) = 25, increment(25) = 26, double(26) = 52
	assert.Equal(t, 52, resultMonoid, "Monoid-based pipeline should match composition (right-to-left)")
}

// TestOperatorType tests the Operator type
func TestOperatorType(t *testing.T) {
	// Create an operator that lifts an int endomorphism to work on the length of strings
	lengthOperator := func(f Endomorphism[int]) Endomorphism[string] {
		return func(s string) string {
			newLen := f(len(s))
			if newLen > len(s) {
				// Pad with spaces
				for i := len(s); i < newLen; i++ {
					s += " "
				}
			} else if newLen < len(s) {
				// Truncate
				s = s[:newLen]
			}
			return s
		}
	}

	// Use the operator
	var op Operator[int, string] = lengthOperator
	doubleLength := op(double)

	result := doubleLength("hello") // len("hello") = 5, 5 * 2 = 10
	assert.Equal(t, 10, len(result), "Operator should transform endomorphisms correctly")
	assert.Equal(t, "hello     ", result, "Operator should pad string correctly")
}

// BenchmarkCompose benchmarks the Compose function
func BenchmarkCompose(b *testing.B) {
	composed := MonadCompose(double, increment)
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		_ = composed(5)
	}
}

// BenchmarkMonoidConcatAll benchmarks ConcatAll with monoid
// TestComposeVsChain demonstrates the key difference between Compose and Chain
func TestComposeVsChain(t *testing.T) {
	double := func(x int) int { return x * 2 }
	increment := func(x int) int { return x + 1 }

	// Compose executes RIGHT-TO-LEFT
	// Compose(double, increment) means: increment first, then double
	composed := MonadCompose(double, increment)
	composedResult := composed(5) // (5 + 1) * 2 = 12

	// MonadChain executes LEFT-TO-RIGHT
	// MonadChain(double, increment) means: double first, then increment
	chained := MonadChain(double, increment)
	chainedResult := chained(5) // (5 * 2) + 1 = 11

	assert.Equal(t, 12, composedResult, "Compose should execute right-to-left")
	assert.Equal(t, 11, chainedResult, "MonadChain should execute left-to-right")
	assert.NotEqual(t, composedResult, chainedResult, "Compose and Chain should produce different results with non-commutative operations")

	// To get the same result with Compose, we need to reverse the order
	composedReversed := MonadCompose(increment, double)
	assert.Equal(t, chainedResult, composedReversed(5), "Compose with reversed args should match Chain")

	// Demonstrate with a more complex example
	square := func(x int) int { return x * x }

	// Compose: RIGHT-TO-LEFT
	composed3 := MonadCompose(MonadCompose(square, increment), double)
	// double(5) = 10, increment(10) = 11, square(11) = 121
	result1 := composed3(5)

	// MonadChain: LEFT-TO-RIGHT
	chained3 := MonadChain(MonadChain(double, increment), square)
	// double(5) = 10, increment(10) = 11, square(11) = 121
	result2 := chained3(5)

	assert.Equal(t, 121, result1, "Compose should execute right-to-left")
	assert.Equal(t, 121, result2, "MonadChain should execute left-to-right")
	assert.Equal(t, result1, result2, "Both should produce same result when operations are in correct order")
}

func BenchmarkMonoidConcatAll(b *testing.B) {
	monoid := Monoid[int]()
	combined := M.ConcatAll(monoid)([]Endomorphism[int]{double, increment, square})
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		_ = combined(5)
	}
}

// BenchmarkChain benchmarks the Chain function
func BenchmarkChain(b *testing.B) {
	chainWithIncrement := Chain(increment)
	chained := chainWithIncrement(double)
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		_ = chained(5)
	}
}