Styled Text Ranges II

Requirements

• Export overwriteStyledText(input, start, end, style) as named and default. input contains text and ranges with start, end, and string style labels. Return a new { text, ranges } with unchanged text and fresh range objects. style is a string, including an empty string, or null to remove styling.
• All ranges and selections use half-open UTF-16 offsets [start, end). Input ranges are valid nonempty ranges inside text, in arbitrary order, and may overlap. When existing ranges overlap, the last range in input order wins at each offset.
• start and end must be finite integers; reject invalid offsets with TypeError. Normalize negative offsets from the end and clamp to text boundaries like String.slice. If normalized end is at or before start, perform no overwrite but still normalize the existing ranges.
• Within the normalized selection, replace every existing style with the supplied style, including previously unstyled gaps. null clears the selection. Outside it, preserve the effective original styles. Do not insert, remove, or otherwise edit text.
• Return ranges sorted by start, with no overlaps or empty ranges. Merge adjacent ranges with exactly equal style labels. Leave unstyled gaps absent; do not merge equal styles across such gaps. Preserve UTF-16 indexing, including surrogate code units.
• Do not mutate input or any nested range. Rich-text rendering, CSS properties, multiple simultaneous style labels per position, grapheme-aware offsets, and malformed input-range validation are outside scope.

Examples

Input: overwriteStyledText({text:"abcdef",ranges:[{start:0,end:6,style:"bold"}]},2,4,"italic");
Output: { text: "abcdef", ranges: [{ start: 0, end: 2, style: "bold" }, { start: 2, end: 4, style: "italic" }, { start: 4, end: 6, style: "bold" }] }

Input: overwriteStyledText({text:"abcd",ranges:[{start:0,end:4,style:"bold"}]},1,3,null);
Output: Only [0,1) and [3,4) remain styled bold.