tools/src/cnf/expr.go

// Copyright 2023 The Dawn & Tint Authors
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package cnf

import (
	"sort"

	"dawn.googlesource.com/dawn/tools/src/container"
)

// Expr is a boolean expression, expressed in a Conjunctive Normal Form.
// Expr is an alias to Ands, which represent all the OR expressions that are
// AND'd together.
type Expr = Ands

// Ands is a slice of Ors
// Ands holds a sequence of OR'd expressions that are AND'd together:
//
//	Ors[0] && Ors[1] && ... && Ors[n-1]
type Ands []Ors

// Ors is a slice of Unary
// Ors holds a sequence of unary expressions that are OR'd together:
//
//	Unary[0] || Unary[1] || ... || Unary[n-1]
type Ors []Unary

// Unary is a negatable variable
type Unary struct {
	// If true, Var is negated
	Negate bool
	// The name of the variable
	Var string
}

// AssumeTrue returns a new simplified expression assuming o is true
func (e Expr) AssumeTrue(o Expr) Expr {
	isTrue := container.NewSet[Key]()
	for _, ors := range o {
		isTrue.Add(ors.Key())
	}

	out := Expr{}
nextAnd:
	for _, ors := range e {
		for _, unary := range ors {
			if isTrue.Contains(unary.Key()) {
				continue nextAnd // At least one of the OR expressions is true
			}
		}
		if isTrue.Contains(ors.Key()) {
			continue // Whole OR expression is true
		}
		out = append(out, ors)
	}
	return out
}

// IsNegative returns true if the expression consists entirely of negated variables.
// This is used by the build generator templates to natively map purely negative
// platform-specific build expressions (e.g. !win) to Bazel's inline
// '//conditions:default' branch (Bazel lacks negation natively).
//
// Example:
//
//	!tint_build_is_win && !tint_build_is_mac => true
//	tint_build_is_win => false
func (e Expr) IsNegative() bool {
	if len(e) == 0 {
		return false
	}
	for _, ors := range e {
		for _, unary := range ors {
			if !unary.Negate {
				return false
			}
		}
	}
	return true
}

// PositiveVarsOfNegative returns the unique, sorted variable names of negated variables in the expression.
// This is used to query the corresponding positive platform targets that must match an empty list []
// inside the generated negative conditional select statements.
//
// Example:
//
//	!tint_build_is_win && !tint_build_is_mac => []string{"tint_build_is_mac", "tint_build_is_win"}
func (e Expr) PositiveVarsOfNegative() []string {
	vars := []string{}
	seen := map[string]bool{}
	for _, ors := range e {
		for _, unary := range ors {
			if unary.Negate && !seen[unary.Var] {
				seen[unary.Var] = true
				vars = append(vars, unary.Var)
			}
		}
	}
	sort.Strings(vars)
	return vars
}