gtfs diff engine

[cka-y]

Closes #2

This pull request introduces the initial release of the GTFS Diff Engine, a memory-efficient Python library and CLI for comparing two GTFS feeds and producing a structured diff conforming to the GTFS Diff v2 schema. The changes include a robust implementation of the core diff logic, a clear public API, a command-line interface, detailed documentation, and supporting scripts for end-to-end usage.

The most important changes are:

Core Functionality and API:

• Implements the core diff logic in engine.py, exposing a single diff_feeds() function that returns a typed Pydantic model representing the diff result. [1] [2]
• Defines the GTFS file schema, supported files, and primary key columns in gtfs_definitions.py, with a helper for primary key lookup.

Command-Line Interface and Tooling:

• Adds a Click-based CLI (gtfs-diff) in cli.py, supporting options for output file, row change cap, pretty-printing, and feed download timestamps.
• Provides a Bash script compare_feeds.sh to automate downloading two GTFS feeds by URL and running the diff tool, with argument parsing and error handling.

Documentation and Examples:

• Expands README.md with a comprehensive overview, installation instructions, usage examples, API reference, supported files table, output schema example, and implementation notes on memory efficiency.
• Adds docs/architecture.md detailing design goals, module structure, streaming diff algorithm, edge case handling, and future improvements.

Packaging and Project Setup:

• Configures packaging with pyproject.toml for installation, development, and test dependencies, and sets up the CLI entry point.
• Adds package versioning and module entry points in __init__.py and __main__.py. [1] [2]

These changes collectively deliver a ready-to-use, well-documented GTFS diff engine suitable for both programmatic and CLI-based workflows.

[jcpitre]

Some testing:

• Went to the extreme and tested it on the biggest feed we have:
  • mdb-2014-202603090029.zip
  • mdb-2014-202603110034.zip

Each is about 1.5GB zipped and over 38 miilion entries in shapes.txt and 50 million of entries in stop_times.txt
I had to stop it as it was working on shapes.txt, using 4GB of memory:

[gtfs-diff 16:31:57 4321MB]   [shapes.txt] scan done in 0.0s — added=50,118,266 deleted=49,537,043 modified=0

Which indicates that there was 50+ million addition and about as many deletion.

Examining shapes.txt, we can see that they use some uuid for the shape_id, e.g.:

0000abe0-5266-475b-808a-5cf929120a80,50.118257595,-5.540823891,229,

That seems to be completely regenerated for each version. That means that trying to do a diff in that kind of dataset is not possible.

• We should make a survey of the dataset we have to see if it's common to have regenerated ids.
• We should consider finding some heuristics that would tell us quickly if a there is a possibility of meaningful comparison between 2 datasets.

[jcpitre]

Tested with STM: mdb-2126-202511111837.zip vs mdb-2126-202511130041.zip.
I spot checked some of the reported differences, and they seems ok, except:

Problems:

• In shapes.txt and stops.txt, there was a significant number (thousands?) of modified lines that were due to coordinates having extra zeros:

          {
            "identifier": {
              "shape_id": "11071",
              "shape_pt_sequence": "150001"
            },
            "raw_value": "11071,45.518332,-73.556250,150001",
            "base_line_number": 35,
            "new_line_number": 35,
            "field_changes": [
              {
                "field": "shape_pt_lon",
                "base_value": "-73.55625",
                "new_value": "-73.556250"
              }
            ]
          }

• In feed_info.txt, the base file had an empty line at the end, and this translated to this jason entry:

       "deleted": [
          {
            "identifier": {
              "feed_publisher_name": "",
              "feed_publisher_url": "",
              "feed_lang": "",
              "feed_start_date": "",
              "feed_end_date": "",
              "feed_version": ""
            },
            "raw_value": ",,,,,",
            "base_line_number": 3
          },

[jcpitre]

Modified code to ignore extra zeros in coordinates.

[emmambd]

@jcpitre Were the big feeds DELFI? I'm curious which datasets they were. IDFM told us their data is 120MB zipped.

[jcpitre]

@jcpitre Were the big feeds DELFI? I'm curious which datasets they were. IDFM told us their data is 120MB zipped.

mdb-2014 is the UK aggregate feed. Size is about 1.5GB zipped. Here are the datasets I used:

• mdb-2014-202603090029.zip
• mdb-2014-202603110034.zip

[davidgamez]

We should do no case-sensitive comparison here

Suggested change

	if a == b:
	if a.lower() == b.lower():