Implementing Dead Code Elimination in Go

This challenge asks you to implement a simplified form of dead code elimination for Go programs. Dead code elimination is a crucial compiler optimization that removes code that will never be executed, thereby reducing program size and potentially improving performance. You will be tasked with identifying and removing statements that are unreachable from any function entry point.

Problem Description

Your goal is to write a Go program that takes a simplified representation of Go code (as described below) and removes all unreachable statements. Unreachable code is defined as any statement that cannot be reached through a sequence of control flow transfers starting from the entry point of any function.

Key Requirements:

1. Input Format: The input will be a slice of Statement structs. Each Statement will have an ID (a unique integer identifier), a Type (e.g., "assignment", "return", "if", "call"), and optionally a TargetID (for jump-like statements) or Condition (for conditional statements).
2. Control Flow:
   • assignment, return, call statements are considered "terminal" in the sense that they directly end a block unless they are followed by another statement.
   • if statements have a Condition and two potential subsequent statement IDs (e.g., IfTrueID, IfFalseID if the condition is true or false respectively). If a Condition is evaluated, the program flow continues to the next statement unless the if statement itself has a TargetID that dictates an unconditional jump.
   • goto statements (represented by Type: "goto") have a TargetID which specifies the ID of the statement to jump to.
3. Entry Points: Assume that the first statement in the input slice is a potential entry point. Any goto statement can also create a new potential entry point if it leads to a statement that was previously considered unreachable.
4. Output: The output should be a slice of Statement structs, containing only the statements that are reachable. The order of the reachable statements in the output should be the same as their original order in the input.

Expected Behavior:

• The program should correctly identify and exclude statements that are never executed.
• It should handle unconditional jumps (goto) and conditional jumps (if) appropriately.
• Statements that are part of a loop but are eventually exited should still be considered reachable.

Edge Cases to Consider:

• A program with no statements.
• A program where all statements are unreachable.
• goto statements that jump to themselves.
• if statements where both branches lead to unreachable code.
• A goto statement pointing to a non-existent statement ID (assume valid input for this challenge, but good to be aware of).

Examples

Example 1:

type Statement struct {
	ID          int
	Type        string
	TargetID    int // Used for goto, or for unconditional jumps within if
	Condition   bool // Used for if statements
	IfTrueID    int // ID of the next statement if Condition is true
	IfFalseID   int // ID of the next statement if Condition is false
}

Input: []Statement{
    {ID: 1, Type: "assignment"},
    {ID: 2, Type: "if", Condition: true, IfTrueID: 3, IfFalseID: 4},
    {ID: 3, Type: "assignment"},
    {ID: 4, Type: "return"},
    {ID: 5, Type: "assignment"}, // Unreachable
}

Output: []Statement{
    {ID: 1, Type: "assignment"},
    {ID: 2, Type: "if", Condition: true, IfTrueID: 3, IfFalseID: 4},
    {ID: 3, Type: "assignment"},
    {ID: 4, Type: "return"},
}
Explanation: Statement 5 is unreachable because the execution flow from statement 1 proceeds to statement 2, then to statement 3 (since Condition is true), then to statement 4 (return), and there's no path to statement 5.

Example 2:

Input: []Statement{
    {ID: 1, Type: "assignment"},
    {ID: 2, Type: "goto", TargetID: 4},
    {ID: 3, Type: "assignment"}, // Unreachable
    {ID: 4, Type: "assignment"},
    {ID: 5, Type: "return"},
}

Output: []Statement{
    {ID: 1, Type: "assignment"},
    {ID: 2, Type: "goto", TargetID: 4},
    {ID: 4, Type: "assignment"},
    {ID: 5, Type: "return"},
}
Explanation: Statement 3 is unreachable because statement 2 is a goto statement that jumps directly to statement 4, bypassing statement 3.

Example 3: Loop and Unconditional Branch within If

Input: []Statement{
    {ID: 1, Type: "assignment"},
    {ID: 2, Type: "if", Condition: true, IfTrueID: 3, IfFalseID: 5}, // Will always go to 3
    {ID: 3, Type: "goto", TargetID: 1}, // Loop back to 1
    {ID: 4, Type: "assignment"}, // Unreachable
    {ID: 5, Type: "return"},
}

Output: []Statement{
    {ID: 1, Type: "assignment"},
    {ID: 2, Type: "if", Condition: true, IfTrueID: 3, IfFalseID: 5},
    {ID: 3, Type: "goto", TargetID: 1},
    {ID: 5, Type: "return"},
}
Explanation: Statement 4 is unreachable. Statement 2 always branches to 3 (due to Condition: true). Statement 3 creates a loop back to 1. Statement 5 is reachable from the false branch of statement 2, but since the condition is true, that path is not taken from statement 2 in this specific instance. However, for the purpose of reachability analysis, we must consider all potential paths. In this simplified model, the `Condition` being `true` means the `IfTrueID` is *always* taken. Statement 4 is never reached through any control flow.

Constraints

• The input slice of Statement structs will contain between 0 and 1000 statements.
• Statement IDs will be unique positive integers.
• TargetID, IfTrueID, and IfFalseID will refer to valid IDs of statements present in the input slice, or be 0 if not applicable or pointing to the end of a block.
• The Type of a statement will be one of: "assignment", "return", "if", "goto", "call".
• For "if" statements, Condition will be a boolean. The IfTrueID and IfFalseID will be provided. If IfTrueID or IfFalseID is 0, it signifies the end of a control flow path (like reaching the next statement in sequence).
• For "goto" statements, TargetID will be the ID of the statement to jump to.
• The input will not contain infinite loops that prevent termination of the reachability analysis itself (e.g., a goto that always jumps to itself without any other exit).

Notes

• This challenge simplifies Go's control flow significantly. For instance, if statements don't have explicit blocks of code associated with them; their flow is determined solely by IfTrueID and IfFalseID. Similarly, there are no explicit basic blocks or complex control flow graphs; we're working with a linear sequence of statements and explicit jumps.
• A good approach would be to use a graph traversal algorithm (like Breadth-First Search or Depth-First Search) to determine reachability. You'll need to maintain a set of visited statements.
• Consider how to handle the start of execution. The first statement is a potential entry point. Any statement that is the target of a goto from a reachable statement also becomes a potential entry point for further exploration.
• Remember that a statement is reachable if there is any path from an entry point to it.