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+# `calendar-stats`, a program to compute statistics from Google calendars.
+
+## Summary
+
+If you keep track of what you spend your time on by creating events in a Google
+Calendar, this program can help you compute some statistics based on those
+events, as long as you adhere to some conventions.
+
+## Features
+
+### Time range selection
+
+By default the program looks at events that start before the beginning of the
+current week, and end before now.
+
+You can change the start week by setting the `-weeks` parameter.
+
+You can set arbitrary start or end by setting the `-start` or `-end` parameters
+to unambiguous values in formats understood by the [dateparse
+library](https://github.com/araddon/dateparse).
+
+Note that events that begin before the start or finish after the end are not
+taken into account, even if they partially run into the selected time frame.
+This may change in the future.
+
+### Time spent per day
+
+The program will count and print how much time overall the events took, per day.
+
+When run against a calendar like the following:
+
+![Calendar screenshot](img/calendar.png)
+
+It will produce the output such as the following:
+```
+$ ./calendar-stats
+[...]
+Time spent per day:
+2023-03-27: 1h45m0s
+2023-03-28: 2h15m0s
+2023-03-29: 1h15m0s
+```
+
+Note that it accounts correctly for overlapping events.
+
+### Time spent per category
+
+If you provide a configuration file which explains how to group events into
+categories, the program will also count how much time was spent on each category.
+In case of overlapping events, the time is accounted proportionally.
+
+With the above calendar and the following config file:
+
+```yaml
+categories:
+- name: mail
+  match:
+  - re: "read e?mail"
+
+- name: meetings
+  match:
+  - re: "meeting"
+
+- name: reviews
+  match:
+  - re: "^review:? "
+```
+
+The output will be:
+
+```
+$ ./calendar-stats
+[...]
+Time spent per category:
+23.8% mail
+52.4% meetings
+19.0% reviews
+Unrecognized:
+2023-03-28T10:00:00+02:00      15m0s  reaad mail
+```
+
+The program will also list unrecognized events, i.e. events that do not match
+any category.
+
+### Event summary corrections
+
+1. Optionally, the program can save unrecognized events into a corrections
+   file.
+2. The user can then change the summary (title) of the events in this file
+   using a text editor.
+3. On subsequent invocation, the program will update event summaries in the
+   calendar based on the edited file.
+
+This is a faster way to retitle multiple events than edit them one by one in
+the Google Calendar interface directly.
+
+Taking the above calendar as an example, the "reaad mail" event summary
+contains a typo.  Running the program with option `--corrections
+corrections.yaml`, the following file will be created:
+
+```yaml
+corrections:
+    - id: 2lb6peh9kscthpiaen2jidjemj
+      summary: reaad mail
+      organizer: Marcin Owsiany
+```
+
+We use a text editor to fix the `summary:` line and run the program again:
+
+```
+$ ./calendar-stats -weeks 5 --corrections corrections.yaml 
+2023/04/15 10:23:20 Updating summary of 1 events...
+2023/04/15 10:23:22 Summaries updated.
+Time spent per day:
+2023-03-27: 1h45m0s
+2023-03-28: 2h15m0s
+2023-03-29: 1h15m0s
+Time spent per category:
+28.6% mail
+52.4% meetings
+19.0% reviews
+```
+
+The summary was updated in Google Calendar, and subsequently all events are recognized.
+The resulting corrections file is now nearly empty:
+
+```yaml
+corrections: []
+```
+
+## How to run the program
+
+You can build your own binary by running `go build .` in the top directory.
+
+From time to time, binaries may be provided in the GitHub [releases](https://github.com/porridge/calendar-stats/releases).
+
+Either way, in order to actually authenticate against Google Calendar API, the program
+needs a `credentials.json` file, which represents an OAuth2 Client. See [instructions
+for generating a client](https://developers.google.com/calendar/api/quickstart/go)
+for a desktop application.
+
+Once you save it in the current directory, the program will authenticate you to Google Calendar
+using a web browser and save a `token.json` file on the first invocation.
+
+See above for examples and use the `-h` parameter to see available options.
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