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Diffstat (limited to 'src/cmd/compile/internal/types2/termlist.go')
-rw-r--r-- | src/cmd/compile/internal/types2/termlist.go | 167 |
1 files changed, 167 insertions, 0 deletions
diff --git a/src/cmd/compile/internal/types2/termlist.go b/src/cmd/compile/internal/types2/termlist.go new file mode 100644 index 0000000000..378ba6b8f4 --- /dev/null +++ b/src/cmd/compile/internal/types2/termlist.go @@ -0,0 +1,167 @@ +// Copyright 2021 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package types2 + +import "bytes" + +// A termlist represents the type set represented by the union +// t1 ∪ y2 ∪ ... tn of the type sets of the terms t1 to tn. +// A termlist is in normal form if all terms are disjoint. +// termlist operations don't require the operands to be in +// normal form. +type termlist []*term + +// allTermlist represents the set of all types. +// It is in normal form. +var allTermlist = termlist{new(term)} + +// String prints the termlist exactly (without normalization). +func (xl termlist) String() string { + if len(xl) == 0 { + return "∅" + } + var buf bytes.Buffer + for i, x := range xl { + if i > 0 { + buf.WriteString(" ∪ ") + } + buf.WriteString(x.String()) + } + return buf.String() +} + +// isEmpty reports whether the termlist xl represents the empty set of types. +func (xl termlist) isEmpty() bool { + // If there's a non-nil term, the entire list is not empty. + // If the termlist is in normal form, this requires at most + // one iteration. + for _, x := range xl { + if x != nil { + return false + } + } + return true +} + +// isAll reports whether the termlist xl represents the set of all types. +func (xl termlist) isAll() bool { + // If there's a 𝓤 term, the entire list is 𝓤. + // If the termlist is in normal form, this requires at most + // one iteration. + for _, x := range xl { + if x != nil && x.typ == nil { + return true + } + } + return false +} + +// norm returns the normal form of xl. +func (xl termlist) norm() termlist { + // Quadratic algorithm, but good enough for now. + // TODO(gri) fix asymptotic performance + used := make([]bool, len(xl)) + var rl termlist + for i, xi := range xl { + if xi == nil || used[i] { + continue + } + for j := i + 1; j < len(xl); j++ { + xj := xl[j] + if xj == nil || used[j] { + continue + } + if u1, u2 := xi.union(xj); u2 == nil { + // If we encounter a 𝓤 term, the entire list is 𝓤. + // Exit early. + // (Note that this is not just an optimization; + // if we continue, we may end up with a 𝓤 term + // and other terms and the result would not be + // in normal form.) + if u1.typ == nil { + return allTermlist + } + xi = u1 + used[j] = true // xj is now unioned into xi - ignore it in future iterations + } + } + rl = append(rl, xi) + } + return rl +} + +// If the type set represented by xl is specified by a single (non-𝓤) term, +// structuralType returns that type. Otherwise it returns nil. +func (xl termlist) structuralType() Type { + if nl := xl.norm(); len(nl) == 1 { + return nl[0].typ // if nl.isAll() then typ is nil, which is ok + } + return nil +} + +// union returns the union xl ∪ yl. +func (xl termlist) union(yl termlist) termlist { + return append(xl, yl...).norm() +} + +// intersect returns the intersection xl ∩ yl. +func (xl termlist) intersect(yl termlist) termlist { + if xl.isEmpty() || yl.isEmpty() { + return nil + } + + // Quadratic algorithm, but good enough for now. + // TODO(gri) fix asymptotic performance + var rl termlist + for _, x := range xl { + for _, y := range yl { + if r := x.intersect(y); r != nil { + rl = append(rl, r) + } + } + } + return rl.norm() +} + +// equal reports whether xl and yl represent the same type set. +func (xl termlist) equal(yl termlist) bool { + // TODO(gri) this should be more efficient + return xl.subsetOf(yl) && yl.subsetOf(xl) +} + +// includes reports whether t ∈ xl. +func (xl termlist) includes(t Type) bool { + for _, x := range xl { + if x.includes(t) { + return true + } + } + return false +} + +// supersetOf reports whether y ⊆ xl. +func (xl termlist) supersetOf(y *term) bool { + for _, x := range xl { + if y.subsetOf(x) { + return true + } + } + return false +} + +// subsetOf reports whether xl ⊆ yl. +func (xl termlist) subsetOf(yl termlist) bool { + if yl.isEmpty() { + return xl.isEmpty() + } + + // each term x of xl must be a subset of yl + for _, x := range xl { + if !yl.supersetOf(x) { + return false // x is not a subset yl + } + } + return true +} |