Enhanced canals: chainable canals, deep canals, and canal irrigation

[acato]

Summary

This PR extends the existing canal system with three major enhancements that make canals a more strategic and versatile infrastructure option:

1. Chainable Canals

Canals are no longer limited to a single tile adjacent to water. Pioneers can now build canal chains across multiple land tiles by placing each new canal adjacent to an existing one. This enables connecting distant water bodies through multi-tile land crossings.

Cost scaling: Each additional tile in the chain increases both build time and gold cost exponentially (2x per connected canal tile), capped at 4x the base cost. This makes long chains expensive but not prohibitively so, encouraging strategic placement.

Implementation: CvPlot::canHaveImprovement() now checks for adjacency to existing canals (not just water). CvPlot::countAdjacentConnectedCanals() performs a BFS through the connected canal chain to determine the chain length for cost calculations. Cost scaling is applied in both CvPlot::getBuildTime() and CvPlayer::getBuildCost().

2. Deep Canals

A new improvement type (IMPROVEMENT_DEEP_CANAL) that is built by upgrading an existing regular canal. Deep canals allow larger lake-capable ships (sloops, schooners, smuggling ships, etc.) to pass through, while regular canals remain limited to coastal ships and fishing boats.

Ship filtering logic:

• Regular canal: Only ships with TerrainImpassable(TERRAIN_OCEAN) (coastal-only ships) and gather boats with harbour space 1 (fishing boats) may enter
• Deep canal: Any ship that can traverse lakes (!TerrainImpassable(TERRAIN_LAKE)) may enter — this includes sloops, schooners, smuggling ships, and similar mid-tier vessels, but excludes ocean-going warships like galleons and frigates

Water area bridge graph: A new subsystem in CvMap tracks which water areas (oceans, lakes) are connected via deep canal chains. This uses a union-find data structure with BFS discovery, rebuilt whenever deep canals are constructed or destroyed. The bridge graph enables the game's Europe-to-lake routing so that lake-capable ships spawning from Europe can reach inland lakes through deep canal chains. Cities adjacent to multiple water areas also create bridges.

Implementation files:

• CvMap.h/cpp: WaterAreaBridge struct, rebuildWaterAreaBridges(), areWaterAreasConnected(), getBridgeLocation(), findWaterAreaRoot() (union-find)
• CvInfos.h/cpp: New bDeepCanal flag on CvImprovementInfo, read from XML
• CvPlot.h/cpp: isDeepCanal(), ship filtering in isValidDomainForAction() and canMoveInto(), canal BFS in getAdjacentSeaArea() for spawn routing
• CvGame.cpp: Bridge graph rebuild on map regeneration and game load
• CvSelectionGroup.cpp: isAmphibPlot() excludes canal tiles so ships aren't blocked from entering them
• XML: IMPROVEMENT_DEEP_CANAL definition, BUILD_DEEP_CANAL build action, bDeepCanal schema tag, art defines, all pioneer-type units given the build ability

3. Canal Irrigation

Canals connected to a freshwater source (lake, large river, or river) via a canal chain enable pioneers to irrigate adjacent desert tiles into flood plains. The freshwater connection is verified by a BFS through the canal chain (CvPlot::isCanalChainConnectedToFreshwater()).

Key design decisions:

• Irrigation is not automatic — a pioneer must manually build the flood plains improvement on each eligible desert tile. This prevents canals from being an overpowered free terrain upgrade
• Only canals connected to freshwater enable irrigation; sea-only canal chains do not. The check uses isAdjacentToFreshwaterCanal() which verifies freshwater connectivity through the entire canal chain
• The freshwater check is applied consistently in both canHaveFeature() (general feature placement) and canBuild() (terraforming validation)

Also included: Ice Hut improvement

A new IMPROVEMENT_ICE_HUT for frozen lake tiles (TERRAIN_ICE_LAKE), providing +2 food. Land units can now enter ice lake tiles (movement validation updated in isValidDomainForAction() and canMoveInto()).

Also included: Ice Hut improvement

A new IMPROVEMENT_ICE_HUT for frozen lake tiles (TERRAIN_ICE_LAKE), providing +2 food. Land units can now enter ice lake tiles (movement validation updated in isValidDomainForAction() and canMoveInto()).

Changed files

DLL Sources

File	Changes
CvPlot.h/cpp	Canal chaining, deep canal checks, freshwater BFS, ship filtering, canal BFS for getAdjacentSeaArea(), cost scaling
CvMap.h/cpp	Water area bridge graph (union-find + BFS), rebuildWaterAreaBridges()
CvInfos.h/cpp	bDeepCanal flag on CvImprovementInfo
CvPlayer.cpp	Canal chain gold cost scaling
CvUnit.cpp	Deep canal ship filtering in canMoveInto(), ice lake land unit access
CvGame.cpp	Bridge graph rebuild on map regen and game load
CvGameTextMgr.cpp	Deep canal tooltip text
CvSelectionGroup.cpp	Exclude canal tiles from isAmphibPlot()
EventTrigger.cpp	Unrelated one-line fix (missing push_back in setRandomNumbers())

XML

File	Changes
CIV4ImprovementInfos.xml	IMPROVEMENT_DEEP_CANAL, IMPROVEMENT_ICE_HUT; canal bNotAllowedNextToSameAsItself set to 0
CIV4TerrainSchema.xml	bDeepCanal element type
CIV4BuildInfos.xml	BUILD_DEEP_CANAL, BUILD_ICE_HUT
CIV4UnitInfos.xml	BUILD_DEEP_CANAL added to all pioneer-type units
CIV4ArtDefines_Improvement.xml	Art define for deep canal (reuses canal art)
XML_AUTO_UTF8_BuildInfo.xml	Text keys for deep canal and ice hut builds (EN/FR/DE)
XML_AUTO_UTF8_ImprovementInfo.xml	Civilopedia and tooltip text for deep canal and ice hut (EN/FR/DE/RU)

Test plan

• Build a canal chain across 3+ land tiles and verify cost caps at 4x base
• Build a regular canal and verify only coastal ships and fishing boats can enter
• Upgrade a canal to deep canal and verify sloops/schooners can traverse it
• Verify galleons/frigates cannot enter deep canals
• Connect a lake to the ocean via deep canals and verify ships spawn/route correctly from Europe to the lake
• Build a canal chain from a lake through desert and verify a pioneer can irrigate adjacent desert into flood plains
• Build a canal chain from the ocean (no freshwater) adjacent to desert and verify irrigation is NOT possible
• Build an ice hut on a frozen lake tile and verify +2 food yield
• Verify land units can walk onto ice lake tiles
• Load a savegame from before these changes and verify it loads correctly
• Check Civilopedia entries for both canal types and ice hut

A test package (DLL + XML files) is included in test-packages/enhanced-canals/ for easy drop-in testing.

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