libutil: Add Pos::getSnippetUpTo(Pos)

src/libutil/position.cc

@@ -116,4 +116,50 @@ void Pos::LinesIterator::bump(bool atFirst)
     input.remove_prefix(eol);
 }
 
+std::string Pos::getSnippetUpTo(const Pos & end) const {
+    assert(this->origin == end.origin);
+
+    if (end.line < this->line)
+        return "";
+
+    if (auto source = getSource()) {
+
+        auto firstLine = LinesIterator(*source);
+        for (auto i = 1; i < this->line; ++i) {
+            ++firstLine;
+        }
+
+        auto lastLine = LinesIterator(*source);
+        for (auto i = 1; i < end.line; ++i) {
+            ++lastLine;
+        }
+
+        LinesIterator linesEnd;
+
+        std::string result;
+        for (auto i = firstLine; i != linesEnd; ++i) {
+            auto firstColumn = i == firstLine ? (this->column ? this->column - 1 : 0) : 0;
+            if (firstColumn > i->size())
+                firstColumn = i->size();
+
+            auto lastColumn = i == lastLine ? (end.column ? end.column - 1 : 0) : INT_MAX;
+            if (lastColumn < firstColumn)
+                lastColumn = firstColumn;
+            if (lastColumn > i->size())
+                lastColumn = i->size();
+
+            result += i->substr(firstColumn, lastColumn - firstColumn);
+
+            if (i == lastLine) {
+                break;
+            } else {
+                result += '\n';
+            }
+        }
+        return result;
+    }
+    return "";
+}
+
+
 }

src/libutil/position.hh

@@ -68,6 +68,8 @@ struct Pos
     bool operator!=(const Pos & rhs) const = default;
     bool operator<(const Pos & rhs) const;
 
+    std::string getSnippetUpTo(const Pos & end) const;
+
     struct LinesIterator {
         using difference_type = size_t;
         using value_type = std::string_view;

tests/unit/libutil/position.cc

@@ -0,0 +1,120 @@
+#include <gtest/gtest.h>
+
+#include "position.hh"
+
+namespace nix {
+
+inline Pos::Origin makeStdin(std::string s)
+{
+    return Pos::Stdin{make_ref<std::string>(s)};
+}
+
+TEST(Position, getSnippetUpTo_0)
+{
+    Pos::Origin o = makeStdin("");
+    Pos p(1, 1, o);
+    ASSERT_EQ(p.getSnippetUpTo(p), "");
+}
+TEST(Position, getSnippetUpTo_1)
+{
+    Pos::Origin o = makeStdin("x");
+    {
+        // NOTE: line and column are actually 1-based indexes
+        Pos start(0, 0, o);
+        Pos end(99, 99, o);
+        ASSERT_EQ(start.getSnippetUpTo(start), "");
+        ASSERT_EQ(start.getSnippetUpTo(end), "x");
+        ASSERT_EQ(end.getSnippetUpTo(end), "");
+        ASSERT_EQ(end.getSnippetUpTo(start), "");
+    }
+    {
+        // NOTE: line and column are actually 1-based indexes
+        Pos start(0, 99, o);
+        Pos end(99, 0, o);
+        ASSERT_EQ(start.getSnippetUpTo(start), "");
+
+        // "x" might be preferable, but we only care about not crashing for invalid inputs
+        ASSERT_EQ(start.getSnippetUpTo(end), "");
+
+        ASSERT_EQ(end.getSnippetUpTo(end), "");
+        ASSERT_EQ(end.getSnippetUpTo(start), "");
+    }
+    {
+        Pos start(1, 1, o);
+        Pos end(1, 99, o);
+        ASSERT_EQ(start.getSnippetUpTo(start), "");
+        ASSERT_EQ(start.getSnippetUpTo(end), "x");
+        ASSERT_EQ(end.getSnippetUpTo(end), "");
+        ASSERT_EQ(end.getSnippetUpTo(start), "");
+    }
+    {
+        Pos start(1, 1, o);
+        Pos end(99, 99, o);
+        ASSERT_EQ(start.getSnippetUpTo(start), "");
+        ASSERT_EQ(start.getSnippetUpTo(end), "x");
+        ASSERT_EQ(end.getSnippetUpTo(end), "");
+        ASSERT_EQ(end.getSnippetUpTo(start), "");
+    }
+}
+TEST(Position, getSnippetUpTo_2)
+{
+    Pos::Origin o = makeStdin("asdf\njkl\nqwer");
+    {
+        Pos start(1, 1, o);
+        Pos end(1, 2, o);
+        ASSERT_EQ(start.getSnippetUpTo(start), "");
+        ASSERT_EQ(start.getSnippetUpTo(end), "a");
+        ASSERT_EQ(end.getSnippetUpTo(end), "");
+        ASSERT_EQ(end.getSnippetUpTo(start), "");
+    }
+    {
+        Pos start(1, 2, o);
+        Pos end(1, 3, o);
+        ASSERT_EQ(start.getSnippetUpTo(end), "s");
+    }
+    {
+        Pos start(1, 2, o);
+        Pos end(2, 2, o);
+        ASSERT_EQ(start.getSnippetUpTo(end), "sdf\nj");
+    }
+    {
+        Pos start(1, 2, o);
+        Pos end(3, 2, o);
+        ASSERT_EQ(start.getSnippetUpTo(end), "sdf\njkl\nq");
+    }
+    {
+        Pos start(1, 2, o);
+        Pos end(2, 99, o);
+        ASSERT_EQ(start.getSnippetUpTo(end), "sdf\njkl");
+    }
+    {
+        Pos start(1, 4, o);
+        Pos end(2, 99, o);
+        ASSERT_EQ(start.getSnippetUpTo(end), "f\njkl");
+    }
+    {
+        Pos start(1, 5, o);
+        Pos end(2, 99, o);
+        ASSERT_EQ(start.getSnippetUpTo(end), "\njkl");
+    }
+    {
+        Pos start(1, 6, o); // invalid: starting column past last "line character", ie at the newline
+        Pos end(2, 99, o);
+        ASSERT_EQ(start.getSnippetUpTo(end), "\njkl"); // jkl might be acceptable for this invalid start position
+    }
+    {
+        Pos start(1, 1, o);
+        Pos end(2, 0, o); // invalid
+        ASSERT_EQ(start.getSnippetUpTo(end), "asdf\n");
+    }
+}
+
+TEST(Position, example_1)
+{
+    Pos::Origin o = makeStdin("  unambiguous = \n    /** Very close */\n    x: x;\n# ok\n");
+    Pos start(2, 5, o);
+    Pos end(2, 22, o);
+    ASSERT_EQ(start.getSnippetUpTo(end), "/** Very close */");
+}
+
+} // namespace nix