1. add cmp.Ordering interface

2. add PartialOrd/Ord/PartialEq/Eq interface
3. add help functions

cmp/cmp.go

@@ -0,0 +1,85 @@
+package cmp
+
+import (
+	"fmt"
+
+	"github.com/go-board/std/cond"
+	"github.com/go-board/std/optional"
+)
+
+// private is a guard for Ordering safety.
+// when Ordering using this, it can't be implemented out this package.
+type private struct{}
+
+// Ordering is a type that represents the ordering of two values.
+type Ordering interface {
+	ordering(private)
+	IsEq() bool
+	IsNe() bool
+	IsLt() bool
+	IsLe() bool
+	IsGt() bool
+	IsGe() bool
+	fmt.Stringer
+}
+
+const (
+	// Less is an Ordering where a compared value is less than another
+	Less = order(-1)
+
+	// Equal is an Ordering where a compared value is equal to another
+	Equal = order(0)
+
+	// Greater is an Ordering where a compared value is greater than another
+	Greater = order(1)
+)
+
+type order int // -1, 0, 1
+
+func (o order) ordering(private) {}
+func (o order) IsEq() bool       { return o == 0 }
+func (o order) IsNe() bool       { return o != 0 }
+func (o order) IsLt() bool       { return o < 0 }
+func (o order) IsLe() bool       { return o <= 0 }
+func (o order) IsGe() bool       { return o >= 0 }
+func (o order) IsGt() bool       { return o > 0 }
+func (o order) String() string {
+	return cond.Ternary(o == 0, "Equal", cond.Ternary(o < 0, "Less", "Greater"))
+}
+
+// PartialEq is a type that represents a partial equality comparison.
+//  see: [PartialEq](https://en.wikipedia.org/wiki/Partial_equivalence_relation)
+type PartialEq[A any] interface {
+	Eq(A) bool
+	Ne(A) bool
+}
+
+// Eq is a type that represents an equality comparison.
+//  see: [Eq](https://en.wikipedia.org/wiki/Equivalence_relation)
+type Eq[A any] interface {
+	PartialEq[A]
+}
+
+// PartialOrd is a type that represents a partially ordered value.
+//  see: [PartialOrd](https://en.wikipedia.org/wiki/Partially_ordered_set)
+type PartialOrd[A any] interface {
+	PartialCmp(A) optional.Optional[Ordering]
+	Lt(A) bool
+	Le(A) bool
+	Gt(A) bool
+	Ge(A) bool
+}
+
+// Ord is a type that represents an ordered value.
+//  see: [Ord](https://en.wikipedia.org/wiki/Total_order)
+type Ord[A any] interface {
+	Eq[A]
+	PartialOrd[A]
+	Cmp(A) Ordering
+}
+
+// OrdFunc is a function that returns an Ordering.
+type OrdFunc[A any] func(lhs A, rhs A) Ordering
+
+// EqFunc is a function that returns a bool.
+type EqFunc[A any] func(lhs A, rhs A) bool

cmp/cmp_test.go

@@ -0,0 +1,80 @@
+package cmp_test
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+	"github.com/go-board/std/cmp"
+	"github.com/go-board/std/slices"
+)
+
+func TestOrdering(t *testing.T) {
+	c := qt.New(t)
+	c.Run("cmp", func(c *qt.C) {
+		c.Assert(cmp.Order(cmp.Less, cmp.Less), qt.Equals, cmp.Equal)
+		c.Assert(cmp.Order(cmp.Less, cmp.Equal), qt.Equals, cmp.Less)
+		c.Assert(cmp.Order(cmp.Less, cmp.Greater), qt.Equals, cmp.Less)
+		c.Assert(cmp.Order(cmp.Equal, cmp.Less), qt.Equals, cmp.Greater)
+		c.Assert(cmp.Order(cmp.Equal, cmp.Equal), qt.Equals, cmp.Equal)
+		c.Assert(cmp.Order(cmp.Equal, cmp.Greater), qt.Equals, cmp.Less)
+		c.Assert(cmp.Order(cmp.Greater, cmp.Less), qt.Equals, cmp.Greater)
+		c.Assert(cmp.Order(cmp.Greater, cmp.Equal), qt.Equals, cmp.Greater)
+		c.Assert(cmp.Order(cmp.Greater, cmp.Greater), qt.Equals, cmp.Equal)
+	})
+
+	c.Run("string", func(c *qt.C) {
+		c.Assert(cmp.Less.String(), qt.Equals, "Less")
+		c.Assert(cmp.Equal.String(), qt.Equals, "Equal")
+		c.Assert(cmp.Greater.String(), qt.Equals, "Greater")
+	})
+}
+
+type user struct {
+	Name string
+	Age  int
+	Tags []string
+}
+
+func (u user) Eq(rhs user) bool {
+	return u.Name == rhs.Name && u.Age == rhs.Age && slices.Equal(u.Tags, rhs.Tags)
+}
+
+func (u user) Ne(rhs user) bool {
+	return !u.Eq(rhs)
+}
+
+func TestCmp(t *testing.T) {
+	c := qt.New(t)
+	c.Run("max", func(c *qt.C) {
+		c.Run("ordered", func(c *qt.C) {
+			c.Assert(cmp.MaxOrdered(1, 2), qt.Equals, 2)
+			c.Assert(cmp.MaxOrdered(2, 1), qt.Equals, 2)
+			c.Assert(cmp.MaxOrdered(2, 2), qt.Equals, 2)
+		})
+		c.Run("by", func(c *qt.C) {
+			userCmp := func(lhs, rhs user) cmp.Ordering {
+				if lhs.Name < rhs.Name {
+					return cmp.Less
+				} else if lhs.Name > rhs.Name {
+					return cmp.Greater
+				} else {
+					return cmp.Equal
+				}
+			}
+			lhs := user{Name: "a", Age: 1, Tags: []string{"a"}}
+			rhs := user{Name: "b", Age: 2, Tags: []string{"b"}}
+			c.Assert(cmp.MaxBy(userCmp, lhs, rhs), qt.DeepEquals, rhs)
+			c.Assert(cmp.MaxBy(userCmp, rhs, lhs), qt.DeepEquals, rhs)
+		})
+	})
+}
+
+func TestEq(t *testing.T) {
+	c := qt.New(t)
+	c.Run("user", func(c *qt.C) {
+		lhs := user{Name: "a", Age: 1, Tags: []string{"a"}}
+		rhs := user{Name: "a", Age: 1, Tags: []string{"a"}}
+		c.Assert(lhs.Eq(rhs), qt.IsTrue)
+		c.Assert(lhs.Ne(rhs), qt.IsFalse)
+	})
+}

cmp/impl.go

@@ -0,0 +1,72 @@
+package cmp
+
+import (
+	"github.com/go-board/std/cond"
+	"github.com/go-board/std/core"
+)
+
+// MaxBy calculates the maximum value of two values by a given function.
+func MaxBy[A any, F func(A, A) Ordering](compare F, lhs, rhs A) A {
+	return cond.Ternary(compare(lhs, rhs).IsGt(), lhs, rhs)
+}
+
+// MaxByKey calculates the maximum value of two values by a key function.
+func MaxByKey[A any, F func(A) K, K Ord[K]](f F, lhs, rhs A) A {
+	keyLhs, keyRhs := f(lhs), f(rhs)
+	return cond.Ternary(keyLhs.Cmp(keyRhs).IsGe(), lhs, rhs)
+}
+
+// Max calculates the maximum value of two values.
+func Max[A Ord[A]](lhs A, rhs A) A {
+	return MaxBy(func(lhs, rhs A) Ordering { return lhs.Cmp(rhs) }, lhs, rhs)
+}
+
+// MaxOrdered calculates the maximum value of two ordered values.
+func MaxOrdered[A core.Ordered](lhs, rhs A) A {
+	return cond.Ternary(lhs > rhs, lhs, rhs)
+}
+
+// MinBy calculates the minimum value of two values by a given function.
+func MinBy[A any, F func(A, A) Ordering](compare F, lhs, rhs A) A {
+	return cond.Ternary(compare(lhs, rhs).IsLt(), lhs, rhs)
+}
+
+// MinByKey calculates the minimum value of two values by a key function.
+func MinByKey[A any, F func(A) K, K Ord[K]](f F, lhs, rhs A) A {
+	keyLhs, keyRhs := f(lhs), f(rhs)
+	return cond.Ternary(keyLhs.Cmp(keyRhs).IsLt(), lhs, rhs)
+}
+
+// Min calculates the minimum value of two values.
+func Min[A Ord[A]](lhs, rhs A) A {
+	return MinBy(func(lhs, rhs A) Ordering { return lhs.Cmp(rhs) }, lhs, rhs)
+}
+
+// MinOrdered calculates the minimum value of two ordered values.
+func MinOrdered[A core.Ordered](lhs, rhs A) A {
+	return cond.Ternary(lhs < rhs, lhs, rhs)
+}
+
+// EqByKey calculates the equality of two values by a key function.
+func EqByKey[A any, F func(A) K, K Eq[K]](f F, lhs, rhs A) bool {
+	lhsKey, rhsKey := f(lhs), f(rhs)
+	return lhsKey.Eq(rhsKey)
+}
+
+// CmpByKey calculates the comparison of two values by a key function.
+func CmpByKey[A any, F func(A) K, K Ord[K]](f F, lhs, rhs A) Ordering {
+	lhsKey, rhsKey := f(lhs), f(rhs)
+	return lhsKey.Cmp(rhsKey)
+}
+
+// Order is a function that returns an Ordering of two Ordering.
+//  result table is:
+//		| lhs\rhs | Less    |  Equal  | Greater |
+//		|---------|---------|---------|---------+
+//		| Less    | Equal   | Less    | Less    |
+//		| Equal   | Greater | Equal   | Less    |
+//		| Greater | Greater | Greater | Equal   |
+func Order(lhs, rhs Ordering) Ordering {
+	lhsInner, rhsInner := lhs.(order), rhs.(order)
+	return cond.Ternary(lhsInner == rhsInner, Equal, cond.Ternary(lhsInner < rhsInner, Less, Greater))
+}