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diff --git a/third_party/wasm2c/src/lexer-source-line-finder.cc b/third_party/wasm2c/src/lexer-source-line-finder.cc
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+++ b/third_party/wasm2c/src/lexer-source-line-finder.cc
@@ -0,0 +1,152 @@
+/*
+ * Copyright 2017 WebAssembly Community Group participants
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "wabt/lexer-source-line-finder.h"
+
+#include <algorithm>
+
+#include "wabt/lexer-source.h"
+
+namespace wabt {
+
+LexerSourceLineFinder::LexerSourceLineFinder(
+    std::unique_ptr<LexerSource> source)
+    : source_(std::move(source)),
+      next_line_start_(0),
+      last_cr_(false),
+      eof_(false) {
+  source_->Seek(0);
+  // Line 0 should not be used; but it makes indexing simpler.
+  line_ranges_.emplace_back(0, 0);
+}
+
+Result LexerSourceLineFinder::GetSourceLine(const Location& loc,
+                                            Offset max_line_length,
+                                            SourceLine* out_source_line) {
+  ColumnRange column_range(loc.first_column, loc.last_column);
+  OffsetRange original;
+  CHECK_RESULT(GetLineOffsets(loc.line, &original));
+
+  OffsetRange clamped =
+      ClampSourceLineOffsets(original, column_range, max_line_length);
+  bool has_start_ellipsis = original.start != clamped.start;
+  bool has_end_ellipsis = original.end != clamped.end;
+
+  out_source_line->column_offset = clamped.start - original.start;
+
+  if (has_start_ellipsis) {
+    out_source_line->line += "...";
+    clamped.start += 3;
+  }
+  if (has_end_ellipsis) {
+    clamped.end -= 3;
+  }
+
+  std::vector<char> read_line;
+  CHECK_RESULT(source_->ReadRange(clamped, &read_line));
+  out_source_line->line.append(read_line.begin(), read_line.end());
+
+  if (has_end_ellipsis) {
+    out_source_line->line += "...";
+  }
+
+  return Result::Ok;
+}
+
+bool LexerSourceLineFinder::IsLineCached(int line) const {
+  return static_cast<size_t>(line) < line_ranges_.size();
+}
+
+OffsetRange LexerSourceLineFinder::GetCachedLine(int line) const {
+  assert(IsLineCached(line));
+  return line_ranges_[line];
+}
+
+Result LexerSourceLineFinder::GetLineOffsets(int find_line,
+                                             OffsetRange* out_range) {
+  if (IsLineCached(find_line)) {
+    *out_range = GetCachedLine(find_line);
+    return Result::Ok;
+  }
+
+  const size_t kBufferSize = 1 << 16;
+  std::vector<char> buffer(kBufferSize);
+
+  assert(!line_ranges_.empty());
+  Offset buffer_file_offset = 0;
+  while (!IsLineCached(find_line) && !eof_) {
+    CHECK_RESULT(source_->Tell(&buffer_file_offset));
+    size_t read_size = source_->Fill(buffer.data(), buffer.size());
+    if (read_size < buffer.size()) {
+      eof_ = true;
+    }
+
+    for (auto iter = buffer.begin(), end = iter + read_size; iter < end;
+         ++iter) {
+      if (*iter == '\n') {
+        // Don't include \n or \r in the line range.
+        Offset line_offset =
+            buffer_file_offset + (iter - buffer.begin()) - last_cr_;
+        line_ranges_.emplace_back(next_line_start_, line_offset);
+        next_line_start_ = line_offset + last_cr_ + 1;
+      }
+      last_cr_ = *iter == '\r';
+    }
+
+    if (eof_) {
+      // Add the final line as an empty range.
+      Offset end = buffer_file_offset + read_size;
+      line_ranges_.emplace_back(next_line_start_, end);
+    }
+  }
+
+  if (IsLineCached(find_line)) {
+    *out_range = GetCachedLine(find_line);
+    return Result::Ok;
+  } else {
+    assert(eof_);
+    return Result::Error;
+  }
+}
+
+// static
+OffsetRange LexerSourceLineFinder::ClampSourceLineOffsets(
+    OffsetRange offset_range,
+    ColumnRange column_range,
+    Offset max_line_length) {
+  Offset line_length = offset_range.size();
+  if (line_length > max_line_length) {
+    size_t column_count = column_range.size();
+    size_t center_on;
+    if (column_count > max_line_length) {
+      // The column range doesn't fit, just center on first_column.
+      center_on = column_range.start - 1;
+    } else {
+      // the entire range fits, display it all in the center.
+      center_on = (column_range.start + column_range.end) / 2 - 1;
+    }
+    if (center_on > max_line_length / 2) {
+      offset_range.start += center_on - max_line_length / 2;
+    }
+    offset_range.start =
+        std::min(offset_range.start, offset_range.end - max_line_length);
+    offset_range.end = offset_range.start + max_line_length;
+  }
+
+  return offset_range;
+}
+
+}  // namespace wabt