Use IndexMap rather than BTreeMap for commodity prices

[alexdewar]

Description

We currently store commodity prices in a BTreeMap. Everywhere else in the code where we need an ordered map, we use IndexMap. While both give consistent ordering when you iterate over them, an IndexMap iterates over items in the same order as they were added (like Python dicts) and a BTreeMap is sorted by the keys.

When I changed commodity prices to use an ordered map in #601, I opted for a BTreeMap on the grounds that we wouldn't always be adding commodities to the map in the same order and having a consistent ordering in different years seemed like a nice property to have. It was a pretty arbitrary choice though. The downside of ordering by keys is that it also sorts by time slice alphabetically, so e.g. winter.afternoon comes before winter.morning. So: commodities possibly ordered slightly better and time slices ordered worse. It doesn't seem worth it.

It doesn't particularly matter, but there aren't really any upsides to using a BTreeMap, so I've changed it to an IndexMap for consistency with the rest of the code. IndexMaps are faster too, although I imagine the performance improvement will be neglible in our case.

This has obviously changed the ordering of the commodity_prices.csv output files, but there shouldn't be any other functional changes.

Type of change

• Bug fix (non-breaking change to fix an issue)
• New feature (non-breaking change to add functionality)
• Refactoring (non-breaking, non-functional change to improve maintainability)
• Optimization (non-breaking change to speed up the code)
• Breaking change (whatever its nature)
• Documentation (improve or add documentation)

Key checklist

• All tests pass: $ cargo test
• The documentation builds and looks OK: $ cargo doc

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.72%. Comparing base (b55c6c1) to head (4f58710).
⚠️ Report is 69 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1040   +/-   ##
=======================================
  Coverage   80.71%   80.72%           
=======================================
  Files          52       52           
  Lines        6924     6926    +2     
  Branches     6924     6926    +2     
=======================================
+ Hits         5589     5591    +2     
  Misses       1063     1063           
  Partials      272      272           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

Pull request overview

This PR replaces BTreeMap with IndexMap for storing commodity prices to maintain consistency with the rest of the codebase and provide insertion-order iteration instead of alphabetically sorted iteration. The change affects only the internal data structure of the CommodityPrices struct without altering the API or functionality.

Key Changes

• Changed CommodityPrices internal storage from BTreeMap to IndexMap
• Updated all related type signatures for iterators
• Updated imports to use indexmap::IndexMap instead of std::collections::BTreeMap

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

File	Description
src/simulation/prices.rs	Core implementation - replaced BTreeMap with IndexMap, updated imports and iterator type signatures
tests/data/*/commodity_prices.csv	Test output files - reordered rows to reflect insertion order instead of alphabetical sorting
tests/data/simple/debug_appraisal_results.csv	Test output file - minor floating-point precision differences in some values

The code changes are clean and well-contained. All necessary type signatures have been updated consistently, and the test data changes reflect the expected reordering from alphabetical (BTreeMap) to insertion order (IndexMap).

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[tsmbland]

Seems fine to me. I was never really sure why we were using BTreeMap to begin with.

The only downside I can think of is that, since we add prices in batches according to pricing strategy, changing the pricing strategy of one or more commodities will end up changing the order of prices in the output file. But I don't think anyone will care too much about that

[dalonsoa]

LGTM, and the explanation makes sense.