The Runtime Complexity (innermost) of the given CpxTRS could be proven to be BOUNDS(n^1, INF).

(0) Obligation:

Runtime Complexity TRS:
The TRS R consists of the following rules:

f(S(x), x2) → f(x2, x)
f(0, x2) → 0

Rewrite Strategy: INNERMOST

(1) DecreasingLoopProof (EQUIVALENT transformation)

The following loop(s) give(s) rise to the lower bound Ω(n¹):
The rewrite sequence
f(S(x), S(x1_1)) →⁺ f(x, x1_1)
gives rise to a decreasing loop by considering the right hand sides subterm at position [].
The pumping substitution is [x / S(x), x1_1 / S(x1_1)].
The result substitution is [ ].

(2) BOUNDS(n^1, INF)

(3) RenamingProof (EQUIVALENT transformation)

Renamed function symbols to avoid clashes with predefined symbol.

(4) Obligation:

Runtime Complexity Relative TRS:
The TRS R consists of the following rules:

f(S(x), x2) → f(x2, x)
f(0', x2) → 0'

S is empty.
Rewrite Strategy: INNERMOST

(5) TypeInferenceProof (BOTH BOUNDS(ID, ID) transformation)

Infered types.

(6) Obligation:

Innermost TRS:
Rules:
f(S(x), x2) → f(x2, x)
f(0', x2) → 0'

Types:
f :: S:0' → S:0' → S:0'
S :: S:0' → S:0'
0' :: S:0'
hole_S:0'1_0 :: S:0'
gen_S:0'2_0 :: Nat → S:0'

(7) OrderProof (LOWER BOUND(ID) transformation)

Heuristically decided to analyse the following defined symbols:
f

(8) Obligation:

Innermost TRS:
Rules:
f(S(x), x2) → f(x2, x)
f(0', x2) → 0'

Types:
f :: S:0' → S:0' → S:0'
S :: S:0' → S:0'
0' :: S:0'
hole_S:0'1_0 :: S:0'
gen_S:0'2_0 :: Nat → S:0'

Generator Equations:
gen_S:0'2_0(0) ⇔ 0'
gen_S:0'2_0(+(x, 1)) ⇔ S(gen_S:0'2_0(x))

The following defined symbols remain to be analysed:
f

(9) RewriteLemmaProof (LOWER BOUND(ID) transformation)

Proved the following rewrite lemma:
f(gen_S:0'2_0(n4_0), gen_S:0'2_0(n4_0)) → gen_S:0'2_0(0), rt ∈ Ω(1 + n4₀)

Induction Base:
f(gen_S:0'2_0(0), gen_S:0'2_0(0)) →_R^Ω(1)
0'

Induction Step:
f(gen_S:0'2_0(+(n4_0, 1)), gen_S:0'2_0(+(n4_0, 1))) →_R^Ω(1)
f(gen_S:0'2_0(+(n4_0, 1)), gen_S:0'2_0(n4_0)) →_R^Ω(1)
f(gen_S:0'2_0(n4_0), gen_S:0'2_0(n4_0)) →_IH
gen_S:0'2_0(0)

We have rt ∈ Ω(n¹) and sz ∈ O(n). Thus, we have irc_R ∈ Ω(n).

(10) Complex Obligation (BEST)

(11) Obligation:

Innermost TRS:
Rules:
f(S(x), x2) → f(x2, x)
f(0', x2) → 0'

Types:
f :: S:0' → S:0' → S:0'
S :: S:0' → S:0'
0' :: S:0'
hole_S:0'1_0 :: S:0'
gen_S:0'2_0 :: Nat → S:0'

Lemmas:
f(gen_S:0'2_0(n4_0), gen_S:0'2_0(n4_0)) → gen_S:0'2_0(0), rt ∈ Ω(1 + n4₀)

Generator Equations:
gen_S:0'2_0(0) ⇔ 0'
gen_S:0'2_0(+(x, 1)) ⇔ S(gen_S:0'2_0(x))

No more defined symbols left to analyse.

(12) LowerBoundsProof (EQUIVALENT transformation)

The lowerbound Ω(n¹) was proven with the following lemma:
f(gen_S:0'2_0(n4_0), gen_S:0'2_0(n4_0)) → gen_S:0'2_0(0), rt ∈ Ω(1 + n4₀)

(13) BOUNDS(n^1, INF)

(14) Obligation:

Innermost TRS:
Rules:
f(S(x), x2) → f(x2, x)
f(0', x2) → 0'

Types:
f :: S:0' → S:0' → S:0'
S :: S:0' → S:0'
0' :: S:0'
hole_S:0'1_0 :: S:0'
gen_S:0'2_0 :: Nat → S:0'

Lemmas:
f(gen_S:0'2_0(n4_0), gen_S:0'2_0(n4_0)) → gen_S:0'2_0(0), rt ∈ Ω(1 + n4₀)

Generator Equations:
gen_S:0'2_0(0) ⇔ 0'
gen_S:0'2_0(+(x, 1)) ⇔ S(gen_S:0'2_0(x))

No more defined symbols left to analyse.

(15) LowerBoundsProof (EQUIVALENT transformation)

The lowerbound Ω(n¹) was proven with the following lemma:
f(gen_S:0'2_0(n4_0), gen_S:0'2_0(n4_0)) → gen_S:0'2_0(0), rt ∈ Ω(1 + n4₀)

(16) BOUNDS(n^1, INF)