ICS Line Parser Stepper¶
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Description¶
The server reads the school calendar as an iCalendar feed, one line at a time. A few rules do most of the work: a line that starts with a space continues the line before it; everything inside a VTIMEZONE block is skipped; a DTSTART with TZID is a local time to convert to UTC; and an all-day DTEND is exclusive, so the last day is one day earlier.
The 14 lines of a small feed adapted from the Cedar Ridge calendar are listed with the eleven possible parser actions and the parser state (inside a time zone or not, the event being built, and the finished events). In explore mode you step through the finished parse. In the quiz you choose the action for each line in turn, and the parser applies the correct one and shows why.
How to Use¶
The MicroSim opens in its starting state and nothing moves until you act.
- Start the 14 lines / Back to explore: Switch between stepping and the quiz.
- Action menu and Submit: Choose one of the actions A to K. One attempt per line.
- Next line / See score: Move on after the feedback.
- Step and Back to start: Explore only: parse one line at a time.
Lesson Plan¶
Learning Objective¶
The learner will execute the iCalendar parsing rules on the 14 lines of a small feed by choosing what the parser does with each line, with at least 12 of the 14 choices correct.
Bloom's Taxonomy level: Apply (execute)
Audience and Duration¶
High school students (grades 9-12) building the Open Study Buddy; about 15 minutes.
Prerequisites¶
property, parameter, VEVENT, VTIMEZONE, line folding, all-day date, exclusive end date, UTC, TZID (all defined in the section "Step Two: An iCalendar Feed, Parsed on the Server" above this block).
Activities¶
- Exploration (5 min): Step through all 14 lines once, reading the parser state after lines 2, 6, 7 and 13.
- Guided practice (5 min): Before line 6, decide what the space at the start of the line means.
- Assessment (5 min): Complete all fourteen lines. Mastery is 12 of 14 correct on the first attempt.
Common Misconceptions Addressed¶
(1) The DTSTART inside a VTIMEZONE is an event. (2) A line that starts with a space is a new property. (3) The end date of an all-day event is its last day. (4) A time with a TZID is already in UTC.
Discussion Questions¶
- Why is the DTSTART on line 2 not an event?
- Why is the last day of Thanksgiving Break November 27 and not 28?
- Is a time with a TZID already in UTC?
Assessment¶
The quiz inside the MicroSim is the evidence of mastery: each item is scored on the first attempt only, the correct answer and its reason are shown after every submission, and the final screen reports the score against the mastery threshold.
Specification¶
Full specification from Chapter 16
Type: microsim
**sim-id:** ics-line-parser-stepper<br/>
**Library:** p5.js<br/>
**Status:** Specified<br/>
**Bloom Level:** Apply<br/>
**Bloom Verb:** execute<br/>
**Learning Objective:** The learner will execute the iCalendar parsing rules on the 14 lines of a small feed by choosing what the parser does with each line, with at least 12 of the 14 choices correct.
**Prerequisites:** property, parameter, VEVENT, VTIMEZONE, line folding, all-day date, exclusive end date, UTC, TZID (all defined in the section "Step Two: An iCalendar Feed, Parsed on the Server" above this block).
**Evidence of Mastery:** For each of the 14 lines, in order, the learner chooses one action from the Actions list, then submits. A choice is correct when it equals the Answer column. Mastery is 12 of 14 correct on the first submission of each line. Stepping through the finished parse in exploration is not evidence.
**Misconceptions:** (1) The DTSTART inside a VTIMEZONE is an event. (2) A line that starts with a space is a new property. (3) The end date of an all-day event is its last day. (4) A time with a TZID is already in UTC.
**Instructional Rationale:** Executing a procedure means following its rules on concrete input. The learner commits each line's action before the parser state is revealed, so a wrong rule shows as a wrong action, and the running state of the parse (inside a time zone or not, the event being built) shows why.
**Content:**
The Actions, always shown:
| Id | Action |
|----|--------|
| A | Start skipping a time-zone definition |
| B | Skip: it is inside a time-zone definition |
| C | Stop skipping |
| D | Start a new event |
| E | Set the title |
| F | Join onto the line before it (a fold) |
| G | Set a timed start, converted to UTC |
| H | Set an all-day first day |
| I | Set the last day (the end date minus one day) |
| J | Set the location |
| K | Finish the event |
The 14 lines, in order. A line that begins with a space is shown with a visible space marker ("␣").
| # | Line | Answer | Why (shown as feedback) |
|---|------|--------|-------------------------|
| 1 | `BEGIN:VTIMEZONE` | A | Everything until the matching END belongs to a time-zone definition. |
| 2 | `DTSTART:19701101T020000` | B | This DTSTART describes a rule of the zone, not an event, so it is skipped. |
| 3 | `END:VTIMEZONE` | C | The zone definition is over; events count again. |
| 4 | `BEGIN:VEVENT` | D | A new event starts with empty fields. |
| 5 | `SUMMARY:TEST: Biology Unit 2 - Gene` | E | The title so far is "TEST: Biology Unit 2 - Gene". The second colon belongs to the value. |
| 6 | `␣tics` | F | A line that starts with a space continues the line before it, and that one space is dropped, even in the middle of a word. The title becomes "TEST: Biology Unit 2 - Genetics". |
| 7 | `DTSTART;TZID=America/Chicago:20261110T100200` | G | 10:02 in Chicago on Nov 10 is CST, UTC minus 6 hours, so 16:02 UTC. |
| 8 | `LOCATION:Lab 130` | J | Plain text property. |
| 9 | `END:VEVENT` | K | The event is complete. Its title begins "TEST:", so its category is test, and its course is Biology. |
| 10 | `BEGIN:VEVENT` | D | A second event starts. |
| 11 | `SUMMARY:NO SCHOOL: Thanksgiving Break` | E | The prefix "NO SCHOOL:" will make its category no-school. |
| 12 | `DTSTART;VALUE=DATE:20261125` | H | VALUE=DATE means all-day: the date is Nov 25 with no time and no time zone. |
| 13 | `DTEND;VALUE=DATE:20261128` | I | The end date is exclusive, so the last day is Nov 27. |
| 14 | `END:VEVENT` | K | The event is complete: Nov 25 to Nov 27. |
After line 14 the sim shows the two finished events: "TEST: Biology Unit 2 - Genetics", category test, course Biology, 2026-11-10 16:02 UTC, location Lab 130; and "NO SCHOOL: Thanksgiving Break", category no-school, all-day, 2026-11-25 to 2026-11-27.
**Provenance:** The line forms come from the real Cedar Ridge feed (`docs/schools/cedar-ridge-high-school/calendar.ics`) and from the parser rules in `src/chapters/16-school-agent-calendar/server/ics_parser.py`. The title of the Biology test is adapted by hand to include a fold. The sim labels the feed "adapted from the Cedar Ridge feed."
**Rules:** Lines are processed in order. While inside a VTIMEZONE (between A and C), every property is skipped. A line that starts with a space or tab is joined to the previous line with its first character removed, before any other rule applies. A DTSTART with a TZID is a local time in that zone and is converted to UTC. A DTSTART with VALUE=DATE is an all-day date and is not converted. A DTEND with VALUE=DATE gives the last day as the date minus one day. A title starting TEST: has category test, DUE: has category due, NO SCHOOL: has category no-school.
**Learner Activity:**
1. The learner reads the Actions list and the feed, then steps through a finished example parse once, seeing the state after each line.
2. The learner starts the 14 lines. Line 1 is highlighted, with the parser state ("outside any event, not in a zone") shown.
3. The learner chooses an action and submits.
4. The sim applies the correct action, updates the parser state and the event being built, and shows the Why text.
5. After line 14, the sim shows the score and the two finished events. The learner should notice that lines 2 and 6 are the two that look like events or properties but are not.
**Feedback:** 14 lines, fixed order, one choice each. Correct: "Correct: <Why>". Incorrect: "Not quite. The parser does this: <the answer's action>. <Why>". The answer is revealed right after each submission, and a running score "n of 14" is shown.
**Starting State:** The 14 lines are shown with line 1 highlighted, nothing parsed, and the question "What does the parser do with this line?"
**Chapter Anchors:** The chapter states that a line starting with a space continues the line before it, that a DTSTART inside a VTIMEZONE is not an event, that all-day events have an exclusive end date, and that a TZID time is converted to UTC.
References¶
- iCalendar - Wikipedia - VEVENT, VTIMEZONE and line folding.
- RFC 5545: Internet Calendaring and Scheduling - IETF - The exact rules for folding, DATE values and TZID.
- Coordinated Universal Time - Wikipedia - Why timed events are stored in UTC.