Create an index shift AST node for ordered set references

Constructs a shifted index reference for ordered sets (e.g., YEAR, SEASON). Used for GAMS lag/lead operators like y-1 (previous year) or y+1 (next year). Negative offset represents backward shift (lag), positive represents forward shift (lead).

Usage

ast_shift(symbol, offset, circular = FALSE, ...)

Arguments

symbol: A character string representing the index symbol (e.g., "y", "ls", "ld").
offset: An integer offset. Negative for backward shift (e.g., -1 for y-1), positive for forward shift (e.g., +1 for y+1).
circular: Logical. FALSE (default): a shift past either end of the set refers to no member, so the term is dropped. TRUE: the shift wraps around the ends of the set (GAMS y--1, y++1).
...: Additional fields stored on the node (passed to new_ast()).

Value

A shift S3 object (subclass of ast).

Examples

ast_shift("y", -1)  # y-1 (previous year)
#> <ast> 
#> [1] "y"  "-1"
ast_shift("y", 1)   # y+1 (next year)
#> <ast> 
#> [1] "y" "1"
ast_shift("ls", -1) # ls-1 (previous season)
#> <ast> 
#> [1] "ls" "-1"