libutil: Add Pos::getSnippetUpTo(Pos)

src/libutil/position.cc

@@ -110,4 +110,50 @@ 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) : std::numeric_limits<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 "";
+}
+
+
 }