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libexpr: Rewrite stripIndentation for indented strings

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This commit should faithfully reproduce the old behavior down to the
bugs. The new code is a lot more readable, all quirks are well
documented, and it is overall much more maintainable.

Change-Id: I629585918e4f2b7d296b6b8330235cdc90b7bade
This commit is contained in:
piegames 2024-10-01 18:41:54 +02:00
parent 765771a355
commit c852ae60da
3 changed files with 183 additions and 116 deletions
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31
src/libexpr/parser/grammar.hh
View file

@ -225,7 +225,8 @@ struct string : _string, seq<
> {};
struct _ind_string {
template<bool Indented, typename... Inner>
struct line_start : semantic, star<one<' '>> {};
template<bool CanMerge, typename... Inner>
struct literal : semantic, seq<Inner...> {};
struct interpolation : semantic, seq<
p::string<'$', '{'>, seps,
@ -233,19 +234,32 @@ struct _ind_string {
must<one<'}'>>
> {};
struct escape : semantic, must<any> {};
/* Marker for non-empty lines */
struct has_content : semantic, seq<> {};
};
struct ind_string : _ind_string, seq<
TAO_PEGTL_STRING("''"),
// Strip first line completely if empty
opt<star<one<' '>>, one<'\n'>>,
star<
list<
seq<
// Start a line with some indentation
// (we always match even the empty string if no indentation, as this creates the line)
_ind_string::line_start,
// The actual line
opt<
plus<
sor<
_ind_string::literal<
true,
plus<
sor<
not_one<'$', '\''>,
seq<one<'$'>, not_one<'{', '\''>>,
seq<one<'\''>, not_one<'\'', '$'>>
not_one<'$', '\'', '\n'>,
// TODO probably factor this out like the others for performance
seq<one<'$'>, not_one<'{', '\'', '\n'>>,
seq<one<'$'>, at<one<'\n'>>>,
seq<one<'\''>, not_one<'\'', '$', '\n'>>,
seq<one<'\''>, at<one<'\n'>>>
>
>
>,
@ -260,7 +274,14 @@ struct ind_string : _ind_string, seq<
seq<one<'\\'>, _ind_string::escape>
>
>
>,
_ind_string::has_content
>
>
>,
// End of line, LF. CR is just ignored and not treated as ending a line
// (for the purpose of indentation stripping)
_ind_string::literal<true, one<'\n'>>
>,
must<TAO_PEGTL_STRING("''")>
> {};

32
src/libexpr/parser/parser-impl1.inc.cc
View file

@ -533,36 +533,48 @@ template<> struct BuildAST<grammar::v1::string> : change_head<StringState> {
struct IndStringState : SubexprState {
using SubexprState::SubexprState;
std::vector<std::pair<PosIdx, std::variant<std::unique_ptr<Expr>, StringToken>>> parts;
std::vector<IndStringLine> lines;
};
template<bool Indented, typename... Content>
struct BuildAST<grammar::v1::ind_string::literal<Indented, Content...>> {
template<> struct BuildAST<grammar::v1::ind_string::line_start> {
static void apply(const auto & in, IndStringState & s, State & ps) {
s.parts.emplace_back(ps.at(in), StringToken{in.string_view(), Indented});
s.lines.push_back(IndStringLine { in.string_view(), ps.at(in) });
}
};
template<bool CanMerge, typename... Content>
struct BuildAST<grammar::v1::ind_string::literal<CanMerge, Content...>> {
static void apply(const auto & in, IndStringState & s, State & ps) {
s.lines.back().parts.emplace_back(ps.at(in), StringToken{ in.string_view(), CanMerge });
}
};
template<> struct BuildAST<grammar::v1::ind_string::interpolation> {
static void apply(const auto & in, IndStringState & s, State & ps) {
s.parts.emplace_back(ps.at(in), s->popExprOnly());
s.lines.back().parts.emplace_back(ps.at(in), s->popExprOnly());
}
};
template<> struct BuildAST<grammar::v1::ind_string::escape> {
static void apply(const auto & in, IndStringState & s, State & ps) {
switch (*in.begin()) {
case 'n': s.parts.emplace_back(ps.at(in), StringToken{"\n"}); break;
case 'r': s.parts.emplace_back(ps.at(in), StringToken{"\r"}); break;
case 't': s.parts.emplace_back(ps.at(in), StringToken{"\t"}); break;
default: s.parts.emplace_back(ps.at(in), StringToken{in.string_view()}); break;
case 'n': s.lines.back().parts.emplace_back(ps.at(in), StringToken{"\n"}); break;
case 'r': s.lines.back().parts.emplace_back(ps.at(in), StringToken{"\r"}); break;
case 't': s.lines.back().parts.emplace_back(ps.at(in), StringToken{"\t"}); break;
default: s.lines.back().parts.emplace_back(ps.at(in), StringToken{in.string_view()}); break;
}
}
};
template<> struct BuildAST<grammar::v1::ind_string::has_content> {
static void apply(const auto & in, IndStringState & s, State & ps) {
s.lines.back().hasContent = true;
}
};
template<> struct BuildAST<grammar::v1::ind_string> : change_head<IndStringState> {
static void success(const auto & in, IndStringState & s, ExprState & e, State & ps) {
e.exprs.emplace_back(noPos, ps.stripIndentation(ps.at(in), std::move(s.parts)));
e.exprs.emplace_back(noPos, ps.stripIndentation(ps.at(in), std::move(s.lines)));
}
};

194
src/libexpr/parser/state.hh
View file

@ -9,10 +9,28 @@ namespace nix::parser {
struct StringToken
{
std::string_view s;
bool hasIndentation = false;
// canMerge is only used to faithfully reproduce the quirks from the old code base.
bool canMerge = false;
operator std::string_view() const { return s; }
};
struct IndStringLine {
// String containing only the leading whitespace of the line. May be empty.
std::string_view indentation;
// Position of the line start (before the indentation)
PosIdx pos;
// Whether the line contains anything besides indentation and line break
bool hasContent = false;
std::vector<
std::pair<
PosIdx,
std::variant<std::unique_ptr<Expr>, StringToken>
>
> parts = {};
};
struct State
{
SymbolTable & symbols;
@ -27,8 +45,7 @@ struct State
void overridesFound(const PosIdx pos);
void addAttr(ExprAttrs * attrs, AttrPath && attrPath, std::unique_ptr<Expr> e, const PosIdx pos);
std::unique_ptr<Formals> validateFormals(std::unique_ptr<Formals> formals, PosIdx pos = noPos, Symbol arg = {});
std::unique_ptr<Expr> stripIndentation(const PosIdx pos,
std::vector<std::pair<PosIdx, std::variant<std::unique_ptr<Expr>, StringToken>>> && es);
std::unique_ptr<Expr> stripIndentation(const PosIdx pos, std::vector<IndStringLine> && line);
// lazy positioning means we don't get byte offsets directly, in.position() would work
// but also requires line and column (which is expensive)
@ -182,98 +199,115 @@ inline std::unique_ptr<Formals> State::validateFormals(std::unique_ptr<Formals>
return formals;
}
inline std::unique_ptr<Expr> State::stripIndentation(const PosIdx pos,
std::vector<std::pair<PosIdx, std::variant<std::unique_ptr<Expr>, StringToken>>> && es)
inline std::unique_ptr<Expr> State::stripIndentation(
const PosIdx pos,
std::vector<IndStringLine> && lines)
{
if (es.empty()) return std::make_unique<ExprString>("");
/* If the only line is whitespace-only, directly return empty string.
* NOTE: This is not merely an optimization, but `compatStripLeadingEmptyString`
* later on relies on the string not being empty for working.
*/
if (lines.size() == 1 && lines.front().parts.empty()) {
return std::make_unique<ExprString>("");
}
/* If the last line only contains whitespace, trim it to not cause excessive whitespace.
* (Other whitespace-only lines get stripped only of the common indentation, and excess
* whitespace becomes part of the string.)
*/
if (lines.back().parts.empty()) {
lines.back().indentation = {};
}
/*
* Quirk compatibility:
*
* » nix-instantiate --parse -E $'\'\'${"foo"}\'\''
* "foo"
* » nix-instantiate --parse -E $'\'\' ${"foo"}\'\''
* ("" + "foo")
*
* Our code always produces the form with the additional "" +, so we'll manually
* strip it at the end if necessary.
*/
const bool compatStripLeadingEmptyString = !lines.empty() && lines[0].indentation.empty();
/* Figure out the minimum indentation. Note that by design
whitespace-only final lines are not taken into account. (So
the " " in "\n ''" is ignored, but the " " in "\n foo''" is.) */
bool atStartOfLine = true; /* = seen only whitespace in the current line */
whitespace-only lines are not taken into account. */
size_t minIndent = 1000000;
size_t curIndent = 0;
for (auto & [i_pos, i] : es) {
auto * str = std::get_if<StringToken>(&i);
if (!str || !str->hasIndentation) {
/* Anti-quotations and escaped characters end the current start-of-line whitespace. */
if (atStartOfLine) {
atStartOfLine = false;
if (curIndent < minIndent) minIndent = curIndent;
}
continue;
}
for (size_t j = 0; j < str->s.size(); ++j) {
if (atStartOfLine) {
if (str->s[j] == ' ')
curIndent++;
else if (str->s[j] == '\n') {
/* Empty line, doesn't influence minimum
indentation. */
curIndent = 0;
} else {
atStartOfLine = false;
if (curIndent < minIndent) minIndent = curIndent;
}
} else if (str->s[j] == '\n') {
atStartOfLine = true;
curIndent = 0;
}
for (auto & line : lines) {
if (line.hasContent) {
minIndent = std::min(minIndent, line.indentation.size());
}
}
/* Strip spaces from each line. */
std::vector<std::pair<PosIdx, std::unique_ptr<Expr>>> es2;
atStartOfLine = true;
size_t curDropped = 0;
size_t n = es.size();
auto i = es.begin();
const auto trimExpr = [&] (std::unique_ptr<Expr> e) {
atStartOfLine = false;
curDropped = 0;
es2.emplace_back(i->first, std::move(e));
for (auto & line : lines) {
line.indentation.remove_prefix(std::min(minIndent, line.indentation.size()));
}
/* Concat the parts together again */
/* Note that we don't concat all adjacent string parts to fully reproduce the original code.
* This means that any escapes will result in string concatenation even if this is unnecessary.
*/
std::vector<std::pair<PosIdx, std::unique_ptr<Expr>>> parts;
/* Accumulator for merging intermediates */
PosIdx merged_pos;
std::string merged = "";
bool has_merged = false;
auto push_merged = [&] (PosIdx i_pos, std::string_view str) {
merged += str;
if (!has_merged) {
has_merged = true;
merged_pos = i_pos;
}
};
const auto trimString = [&] (const StringToken t) {
std::string s2;
for (size_t j = 0; j < t.s.size(); ++j) {
if (atStartOfLine) {
if (t.s[j] == ' ') {
if (curDropped++ >= minIndent)
s2 += t.s[j];
}
else if (t.s[j] == '\n') {
curDropped = 0;
s2 += t.s[j];
} else {
atStartOfLine = false;
curDropped = 0;
s2 += t.s[j];
}
} else {
s2 += t.s[j];
if (t.s[j] == '\n') atStartOfLine = true;
}
}
/* Remove the last line if it is empty and consists only of
spaces. */
if (n == 1) {
std::string::size_type p = s2.find_last_of('\n');
if (p != std::string::npos && s2.find_first_not_of(' ', p + 1) == std::string::npos)
s2 = std::string(s2, 0, p + 1);
auto flush_merged = [&] () {
if (has_merged) {
parts.emplace_back(merged_pos, std::make_unique<ExprString>(std::string(merged)));
merged.clear();
has_merged = false;
}
es2.emplace_back(i->first, std::make_unique<ExprString>(std::move(s2)));
};
for (; i != es.end(); ++i, --n) {
std::visit(overloaded { trimExpr, trimString }, std::move(i->second));
for (auto && [li, line] : enumerate(lines)) {
/* Always merge indentation, except for the first line when compatStripLeadingEmptyString is set (see above) */
if (!compatStripLeadingEmptyString || li != 0) {
push_merged(line.pos, line.indentation);
}
/* If this is a single string, then don't do a concatenation. */
if (es2.size() == 1 && dynamic_cast<ExprString *>(es2[0].second.get())) {
return std::move(es2[0].second);
for (auto & val : line.parts) {
auto &[i_pos, item] = val;
std::visit(overloaded{
[&](StringToken str) {
if (str.canMerge) {
push_merged(i_pos, str.s);
} else {
flush_merged();
parts.emplace_back(i_pos, std::make_unique<ExprString>(std::string(str.s)));
}
return std::make_unique<ExprConcatStrings>(pos, true, std::move(es2));
},
[&](std::unique_ptr<Expr> expr) {
flush_merged();
parts.emplace_back(i_pos, std::move(expr));
},
}, std::move(item));
}
}
flush_merged();
/* If this is a single string, then don't do a concatenation.
* (If it's a single expression, still do the ConcatStrings to properly force it being a string.)
*/
if (parts.size() == 1 && dynamic_cast<ExprString *>(parts[0].second.get())) {
return std::move(parts[0].second);
}
return std::make_unique<ExprConcatStrings>(pos, true, std::move(parts));
}
}