Spec #38.1: Implement OPTION BASE statement

[treytomes]

Description

Parse and store array base index (0 or 1) per ECMA-55 specification.

OPTION BASE sets the lower bound for all array subscripts. Must be called before any arrays are allocated.

ECMA-55 References

• ECMA55-ARR-001: OPTION BASE 0 or OPTION BASE 1
• ECMA55-ARR-002: Must appear before any DIM statements

Acceptance Criteria

• Parse valid OPTION BASE 0 and OPTION BASE 1 statements
• Reject invalid values (OPTION BASE 2, -1, etc.)
• Reject multiple OPTION BASE statements
• Default to BASE 0 if not specified
• Error if OPTION BASE after array allocation
• All tests pass
• Test coverage ≥ 80%

Tests to Write First

// Valid cases
OPTION BASE 0 → sets base to 0
OPTION BASE 1 → sets base to 1

// Invalid cases
OPTION BASE 2 → syntax error
OPTION BASE -1 → syntax error
OPTION BASE 0, then OPTION BASE 1 → error (duplicate)

// Default behavior
No OPTION BASE → defaults to 0

// Timing error
DIM A(10), then OPTION BASE 1 → error (too late)

Implementation Tasks

• Create OptionBaseStatementParser.cs
• Add ArrayBase property to IEnvironment
• Add validation for BASE value (0 or 1 only)
• Track whether BASE has been set
• Validate BASE not changed after array allocation

Files Affected

• src/ECMABasic.Domain/IEnvironment.cs
• src/ECMABasic.Application/Parsers/OptionBaseStatementParser.cs
• src/ECMABasic.Application/EnvironmentBase.cs
• test/ECMABasic.Test/OptionBaseStatementTests.cs

Parent Issue

Part of #38 - Implement DIM and OPTION BASE for array support

Estimated Effort

2-3 hours

Dependencies

None - this is the foundation for array support