Mepo: Fast, simple, and hackable OSM map viewer for Linux. Designed with the Pinephone & mobile linux in mind; works both offline and online.

#Mepo

Note: Mepo is currently in active development and while functional, largely intended currently for early adopters (prior to the 1.0 release). See below for the high-level goals, current state of Mepo, the path to 1.0, and information on contributing!

Mepo is a fast, simple, and hackable OSM map viewer for desktop & mobile Linux devices (like the PinePhone, Librem 5 etc.) and both environment's various user interfaces (Wayland & X inclusive). Mepo works both offline and online, features a minimalist both touch/mouse and keyboard compatible interface, and offers a UNIX-philosophy inspired underlying design, exposing a powerful command language called Mepolang capable of being scripted to provide things like custom bounding-box search scripts, bookmarks, and more.

#Goals

Fast & usable in resource-constrained environments:
- Mepo launches quickly and runs quickly on the PinePhone and other resource-constrained devices.
- Mepo renders using SDL which keeps things fast and lightweight; also as a bonus: portable.
- Built in a non GC'd language (Zig) with an aim toward careful memory usage and allocations/deallocations
UNIX-philosophy inspired design - scriptability via mepolang:
- Mepo's UI is built to do one thing well: download & render maps. Extra functionality is enabled via its command language / API (Mepolang).
- Reduces overall application logic complexity, handing over to shell-scripting integral features like map bounding-box searches, location search lookup, bookmarking, dropping pins, routing, rebinding keys, and more.
- Uses single abstraction (central pin API via Mepolang) for indicating and placing user-defined coordinates graphically on the map (e.g. same mechanism used generically between POI searches, bookmarks, routing, etc.)
- Existing OSM tools for search & routing like the Nominatim, Overpass, and GraphHopper APIs are integrated through bundled shell scripts; customize to your heart's content or write your own scripts for custom integrations.
- Bundled scripts utilize existing tools like dmenu for user input and list selection rather then implementing application-specific input logic.
Offline operation as a first-class feature:
- Downloading of maps for later offline use can be done non-interactively through a command-line flag.
- Users can download based on a bounding-box or a user-specified radius from a specific point for multiple zoom levels.
- Offline usage is a primary usecase and should be treated as such, we can't assume a user is always online.
Supports touch AND keyboard-oriented operation:
- A map application must of course be usable with a mouse / touch, but the keyboard as a tool for map navigation has been overlooked in in map applications.
- Provides vi-like (& customizable) keybindings out-of-the-box.
- Should be usable in touch-oriented environments like the PinePhone and similar where a physical keyboard isn't present.

#High-level overview of currently implemented features

Fast SDL-based rendering of OSM map tiles
Downloading of OSM map tiles from user-specified source (default: OSM Maps)
Minimal command-language for Mepolang in which all core-functionality built out
Pan via mouse, double tap to zoom, right click or triple left click to zoom out
Vi-like keys (though customizable) for panning, zooming, center on mouse, etc.
Navigate to regions or addresses via shell-script (Nominatim API based)
Drop pins / search for POI via shell-script (Overpass API based)
Cycle through pins using hotkeys (vim-like n/p, though customizable)
Switch tile URL source via shell-script (toggle between OSM, Bike maps, Google)
Dynamic tile caching to filesystem / offline-maps once downloaded
Run API commands (Mepolang) via command prompt (dmenu) or via STDIN (non-continual)
Basic pinch-to-zoom via SDL multitouch API on PinePhone / mobile
Piping of current state (lat/lon & bbox) to scripts in shellpipe command
Basic non-interactive tile downloading based on viewport bounding-box

#Roadmap to 1.0 release

Milestone 1: Overall Stability (Complete: 10/31/2021, tag: 0.1)
- Done:
  - Improve current stability of the application and resolve intermittent crashes related to tile downloading.
  - Implement tile downloading retry logic and handle case where user is offline gracefully
  - Error check invalid PNG tile data received from tile servers & loaded from filesystem.
  - Rework consistency of allocator passing (e.g. remove ad hoc allocators, ensure all allocations based on using top-level allocator via arena allocation).
  - Rework blitting logic to be more intelligent about not drawing off-screen entities & pins.
  - Implement LIFO for downloading queue: viewport tiles should always take priority before long-queued off-screen tiles.
  - Refactor error handling logic & use utility function rather than zig dbg.
  - Implement togglable text debug message overlay to provide user feedback right in UI when operations like shell pipe fail.
  - Add togglable help text overlay to show current results of bind_key (keybindings table) mapping in UI.
  - Fix bug where negative latitude values fail to round-trip between lat-to-y / y-to-lat conversion functions.
  - Implement debug mode preference or similar to control verbosity of debugging STDERR messages.
  - Resolve existing memory leaks in the application.
  - Provide an initial tag for package maintainers (0.1).
Milestone 2: Offline Mode & Non-Interactive Downloading (Complete: 12/06/2021, tag: 0.2)
- Done:
  - Add error handling logic to existing bounding-box based Mepolang download command to ensure valid bounding-box provided by user.
  - Rework main application / boot logic to enable fully non-interactive mode (free of SDL/Video context) to allow for CLI-based downloading of tiles.
  - Remove SDL delay from TileCache downloading logic / properly wait on file descriptors instead using curl_multi_wait.
  - Add a shell script to allow downloading either interactively or non-interactively based on Nominatim search query result so you can say download an entire city for example.
  - Parameterize max number of concurrent downloads as a setting accessible via prefset.
  - Add configuration parameter to set an offline mode; when set to true all tile downloading logic should no-operation for the privacy and bandwidth conscience.
  - Rework UI logic to adjust downloading bar to not overlap with pin details overlay.
  - Create Mepolang command to selectively clear download cache so tiles can be updated if outdated. (Note: decided to implemented expiry seconds preference instead)
  - Add radius-based download command allowing user to download tiles based on coordinate with a given radius (km distance) for specified zoom-levels.
  - Add distance to lat-lon conversion functions to assist in above (along with unit tests) and to later be used for measuring distance between pins in ordered groups.
Milestone 3: Powerful, Robust Pin API (In-progress)
- Done:
  - Correct pin-selection mode cycling to properly respect delta values.
  - Allow ability to associate metadata with pins.
  - Fix existing pin bugs (e.g. title not showing properly & brittleness).
  - Allow pin groups to be either ordered or unordered, ordered pin groups show in UI with connecting lines (thus enabling lightweight navigation).
  - Implement distinction between structural pins and informational pins for ordered pin groups wherein structural pins are the nodes in a way and informational pins can be used for navigational cues.
  - Add script to add pins based on Nominatim bounding box results in addition to the existing Overpass pin script.
  - Implement bind_click Mepolang API command to run arbitrary Mepolang on clicking specific button (left, right click) 1-n times. Will enable more sophisticated scripts (e.g. such as click to define points in custom route / ordered pin group) in future.
  - Add support for click pin to activate.
  - Add pin groups and extend API as such so user can visually distinguish groups; for example one pin group may be bookmarks, another for a search, another for current location.
  - Implement command for removing specific pins based on handle.
- Todo:
  - Add ability to cycle through pins based on both: viewport visible pins and all pins.
  - Add script to convert XML Relation & Way from OSM API into ordered pin group(s) for testing purposes. Also should help for transit-specific and other overlays etc.
Milestone 4: Wayland & Mobile Support
- Todo:
  - Improve support for pinch-to-zoom using SDL multitouch gesture API to be more reliable and test on PinePhone & mobile devices.
  - Rework pinch-to-zoom to not be hardcoded but to instead use new Mepolang command (bind_gesture).
  - Add support for rotate gesture via SDL multitouch gesture API and expose as part of bind_gesture command.
  - Currently scripts for location search, bounding-box search, and similar use dmenu (X-based); integrate wofi, bemenu, or a similar tool for Wayland. Dynamically determine which menuing system to use in scripts.
  - Allow binding left hand/right hand side of pin overlay via new (bind_button) command and set to pin_cycle by default. Thus enabling touch-based pin-cycling for navigation.
  - Build out context-menu to be used in mobile-context allowing functionality triggered on desktop via keybindings for search etc. to be simply triggered on mobile.
  - Add to default Mepolang configuration trigger to open context-menu either via bind_gesture or bind_click.
  - Perform testing and make fixes as necessary on most common UIs on postmarketOS. Primarily test and ensure compatibility with Phosh (mobile Wayland), Swmo (mobile Wayland), Sxmo (mobile X), Sway (desktop Wayland), and i3 (desktop X) as these 5 environments encompass large userbases and represent the 4 different combinations of mobile/desktop & X/Wayland that should all have first-class support.
  - Rework SDL defaults (window expose, winch, default sizing etc.) to ensure greatest cross-compatibility between environments.
Milestone 5: UI Niceties
- Todo:
  - Rework main application event loop to handle STDIN file descriptor in parallel to current SDL events processing. So commandline of application can thus be used non-interactively / scripted. Also helpful for debugging.
  - Gracefully handle switching tile sources (e.g. purge cache & reload). Should be able to switch between OSM/bikemaps/stamen etc. without glitches.
  - Add support for pasting / copying current coordinates from the system clipboard and ensure compatibility in both Wayland & X.
  - Add visual per-tile-downloading progress-indicators.
  - Improve bottom status bar display to show bandwidth usage / download of tiles, disk, and memory usage.
  - Integrate gpsd into a script to update user's current location pin and fallback to geoclue2 as needed.
  - Add routing script based on GraphHopper, OpenRouteService, or similar to parse GeoJSON LineString API response into an ordered pin group. Thus allowing for point-to-point navigation.
  - Refine existing Overpass search script (provide better recommendations illustrating OSM tag-based flexibility).
  - Add example bookmarking script based on pin API.
  - Finalize Mepolang 1.0 API and add error checking so things don't crash when accessing invalid union tag etc (improve robustness of mepolang parser).
  - Implement save state / restore state Mepolang API command that dumps current state to file (as Mepolang) / and can restore via loading file.
  - Make shell pipe non-blocking (e.g. don't freeze UI while waiting for result) or otherwise implement timeout so that runaway scripts don't freeze application.
Milestone 6: Documentation & Packaging
- Todo:
  - Add manpage documenting Mepolang commands and basic usage.
  - Add markdown-based documentation to explain mepolang commands, usage in parity with man page.
  - Document existing scripts used for search, routing, etc. and add notes about how these scripts could be used offline by running Nominatim, Overpass, GraphHopper, etc. locally.
  - Document examples on how to write custom scripts interfacing with mepolang.
  - Modify internals of Mepolang to include documentation directly in structs so that Mepolang API documentation can be automatically generated as a webpage for end-users.
  - Modify internals of prefset / preferences table to automatically generate documentation directly from code / structs.
  - Create Alpine package and add to pmOS postmarketos-ui-* frontends.
  - Package-split current scripts/ directory to allow separate packages for mepo and mepo-scripts. The former only will contain the base application (Zig) executable; the latter will contain auxiliary scripts.
  - Work with maintainers of mobile Linux (& PinePhone) UIs to get Mepo integrated properly.
  - Tag 1.0 release.

#Funding

NLnet has kindly awarded a grant to support Mepo's development efforts as part of the NGI0 Discovery Fund under grant agreement No 825322. This funding will enable the financial support for implementing of the roadmap to reach 1.0 as seen above. We are very thankful to NLnet for accepting our application and making this development possible!

#Contributing

Mepo is surely welcome to contributors who have experience or interest in mobile Linux, OSM, and/or Zig.