# How gc_play_by_play() and wsc_play_by_play() Work

## Overview

`gc_play_by_play()` and `wsc_play_by_play()` return the same cleaned public play-by-play schema, but they do not start from the same raw feed. `gc_play_by_play()` starts from the GameCenter play-by-play feed. `wsc_play_by_play()` starts from the World Showcase feed and uses GameCenter metadata to keep the output aligned with the same roster, team, and HTML report context. The pipeline is intentionally source-aware:

- The API play-by-play is the source of truth for event order, event identity, and raw `situationCode`.
- The HTML play-by-play report is the primary source for on-ice player identities.
- `shift_chart()` is not used to populate on-ice player IDs inside these functions. It is a separate downstream source for shift timing via `add_shift_times()`.

This article walks through the process step by step.

## Step 1: Fetch the raw sources.

For a single game, the functions fetch the raw API play-by-play plus the HTML play-by-play report. `wsc_play_by_play()` also fetches the WSC play-by-play feed itself. Those requests are now issued in parallel, because network latency dominates total runtime much more than the in-memory cleaning steps do.

    gc  <- nhlscraper::gc_play_by_play(2023030417)
    wsc <- nhlscraper::wsc_play_by_play(2023030417)

The HTML report is fetched for every game because it is the only source that consistently exposes the full on-ice player sets.

## Step 2: Standardize the raw play-by-play feed.

Once the raw feed is downloaded, the package standardizes the columns before doing any enrichment. That includes:

- moving the `gameId` into the table
- flattening nested API fields into a consistent tabular shape
- renaming public-facing columns such as `periodNumber`, `eventTypeCode`, and `eventTypeDescKey`
- filling obvious source omissions such as missing `shootingPlayerId` values on goal rows when the scorer is known

The aim here is to make the downstream logic operate on one internal structure, even when the upstream feed formats differ.

## Step 3: Repair obviously impossible event ordering.

The API play-by-play remains authoritative, but not every upstream `sortOrder` is logically consistent. Before any HTML matching happens, the pipeline repairs clear boundary mistakes. The guiding principle is conservative: only fix sequences that are plainly impossible from the game clock and event context.

Examples:

- no event can happen between `period-start` and the opening `faceoff`
- illogically ordered boundary faceoffs are dropped
- blocked shots keep the API event identity, but their directional perspective is normalized to the shooting team so they behave like the other shot events

This matters because HTML matching becomes much more reliable once the API timeline itself is internally coherent.

## Step 4: Derive the game-state columns from `situationCode`.

The raw `situationCode` is parsed into the public state columns:

- `homeIsEmptyNet`
- `awayIsEmptyNet`
- `homeSkaterCount`
- `awaySkaterCount`
- `manDifferential`
- `strengthState`

These are the public summary columns that describe the state context of the play. The raw `situationCode` itself is kept in the output as-is. That preserves the upstream source record even when the package later concludes that the HTML report indicates a more realistic on-ice identity set.

## Step 5: Add coordinate and shot-context enrichment.

After the structural cleanup, the play-by-play gets the geometric and shot-context features used elsewhere in the package. That includes:

- normalized rink coordinates
- shot distance
- shot angle
- rush flags
- rebound flags
- rebound-creation flags
- cumulative goal, shot, Fenwick, and Corsi counts from the event stream

These features are derived from the cleaned API event log, not from the HTML report.

## Step 6: Parse the HTML play-by-play report.

The HTML report is parsed into a second event table that contains:

- the event type
- the period and clock
- team ownership context
- key player signatures used for matching
- the home and away on-ice player sets

The parser also handles the known HTML-side quirks, including the fact that the HTML report records blocked shots from the defending perspective while the package standardizes blocked shots from the shooting perspective.

## Step 7: Match HTML rows back to API rows.

The package does not join HTML to API rows only on time. It builds a richer event signature on both sides and uses that to align the reports. The matching logic uses combinations of:

- period
- elapsed seconds
- event type
- event-owning team
- primary event actors
- supporting player signatures

This step is where the earlier ordering repair pays off. The cleaner the API event sequence is, the safer the HTML match becomes.

## Step 8: Decide whether an HTML on-ice row is safe to use.

A matched HTML row is not accepted blindly. The package checks whether the HTML on-ice set is compatible with the API-side understanding of the play. There are three main acceptance paths:

1. The HTML player counts match the raw `situationCode`.
2. The HTML player counts match a rules-based strength reconstruction from the API penalty sequence.
3. The HTML counts disagree, but the event actors clearly sit inside the HTML on-ice sets, which makes the row useful for player identity even if the upstream count context is off.

That third path is the deliberate compromise between strict consistency and recording what most likely happened on the ice.

## Step 9: Reconstruct known strength-state mistakes conservatively.

Some API stretches clearly carry stale or implausible manpower state even though the event stream itself gives enough information to reconstruct the expected skater counts. The reconstruction logic handles narrow cases such as:

- active penalties
- double minors
- goal-based minor releases
- coincidental major suppression
- delayed-penalty and late empty-net situations

When the package has enough evidence to trust the rules-based reconstruction, it updates the derived strength context columns. The raw `situationCode` still remains untouched.

## Step 10: Populate on-ice player IDs.

Once a matched HTML row is accepted, the package writes the scalar on-ice player ID columns into the play-by-play row. That includes:

- `homeGoaliePlayerId` and `awayGoaliePlayerId`
- `homeSkater1PlayerId` through `homeSkater5PlayerId` by default, with extra skater slots added only when the game needs them
- `awaySkater1PlayerId` through `awaySkater5PlayerId` by default, with extra skater slots added only when the game needs them
- the corresponding `...For` and `...Against` columns

The base schema tracks the standard five skaters. If the HTML report shows an extra attacker or any other overflow row, the package expands dynamically to `skater6`, `skater7`, `skater8`, and so on instead of truncating the row.

## Step 11: Handle one-on-one and delayed-penalty edge cases.

Two edge-case families need their own rules.

### Shootouts and penalty shots

Rows with one-on-one states such as `0101` and `1010` are allowed to populate only the shooter and the defending goalie. The HTML report typically shows only those players, which is correct for the event.

### Unmatched delayed-penalty rows

Some supported delayed-penalty rows do not appear in the HTML report at all. In those cases, the package can backfill the on-ice player IDs from the nearest prior populated row in the same period when:

- the state signature is unchanged
- the time gap is very small
- the prior row already has a compatible populated on-ice set

This fixes cases where the HTML report skips the delayed-penalty marker but clearly preserves the same live-play skaters immediately before the whistle.

## Step 12: Finalize the public schema.

The last step is to expose the cleaned public-facing schema and hide the internal staging details. Both `gc_play_by_play()` and `wsc_play_by_play()` return one row per event with:

- the same core event columns
- the same strength and on-ice player ID columns
- the same cumulative game-state columns

The only intentional difference is source-specific metadata such as `utc` in the WSC output and GameCenter clip fields in the GC output.

## How `shift_chart()` Fits In

`shift_chart()` is related, but it solves a different problem. It provides shift windows, not event identities. In practical use:

    pbp    <- nhlscraper::gc_play_by_play(2023030417)
    shifts <- nhlscraper::shift_chart(2023030417)
    pbp_with_shift_times <- nhlscraper::add_shift_times(pbp, shifts)

This is why the package keeps the HTML play-by-play report as the primary on-ice identity source inside `gc_play_by_play()` and `wsc_play_by_play()`, while `shift_chart()` remains the right tool for shift-timing context after the play-by-play is already built.

## Practical Summary

If you want the shortest mental model, it is this:

- start from the API play-by-play
- repair only the event-order and strength mistakes that are logically supportable
- use the HTML report to recover the actual players on the ice
- preserve the raw `situationCode`, but do not let it block clearly useful player identity rows
- use `shift_chart()` later when you need shift timing rather than event-level player identity

That balance is what lets the final play-by-play stay both practical and auditable.