Add overload for single particle momentum

src/interfaces/phase_space_point.jl

@@ -65,7 +65,11 @@ function momentum(psp::AbstractPhaseSpacePoint, dir::ParticleDirection, n::Int)
 end
 
 function _momentum_helper(particles::Tuple{}, species::SPECIES, n::Val{N}) where {SPECIES,N}
-    throw(BoundsError("requested $species momentum is not in this phase space point"))
+    throw(
+        BoundsError(
+            "momentum(): requested $species momentum is not in this phase space point"
+        ),
+    )
 end
 
 function _momentum_helper(
@@ -110,6 +114,64 @@ function momentum(
     return _momentum_helper(particles(psp, dir), species, n)
 end
 
+function _assert_one_particle(
+    particles::Tuple{AbstractParticleStateful{DIR,SPECIES,EL},Vararg},
+    dir::DIR,
+    species::SPECIES,
+    n::Val{1},
+) where {DIR,SPECIES,EL}
+    throw(
+        InvalidInputError(
+            "momentum(): more than one $dir $species exists in this phase space point"
+        ),
+    )
+end
+
+function _assert_one_particle(
+    particles::Tuple{}, dir::DIR, species::SPECIES, n::Val{0}
+) where {DIR,SPECIES}
+    throw(
+        InvalidInputError("momentum(): no $dir $species exists in this phase space point")
+    )
+end
+
+function _assert_one_particle(
+    particles::Tuple{}, dir::DIR, species::SPECIES, n::Val{1}
+) where {DIR,SPECIES}
+    return nothing
+end
+
+function _assert_one_particle(
+    particles::Tuple{AbstractParticleStateful{DIR,SPECIES,EL},Vararg},
+    dir::DIR,
+    species::SPECIES,
+    n::Val{0},
+) where {DIR,SPECIES,EL}
+    return _assert_one_particle(particles[2:end], dir, species, Val(1))
+end
+
+function _assert_one_particle(
+    particles::Tuple{AbstractParticleStateful{DIR,SPECIES,EL},Vararg},
+    dir::DIR2,
+    species::SPECIES2,
+    n::Val{N},
+) where {DIR,SPECIES,EL,DIR2,SPECIES2,N}
+    return _assert_one_particle(particles[2:end], dir, species, n)
+end
+
+"""
+    momentum(psp::AbstractPhaseSpacePoint, dir::ParticleDirection, species::AbstractParticleType)
+
+Returns the momentum of the particle in the given [`AbstractPhaseSpacePoint`](@ref) with `dir` and `species`, *if* there is only one such particle. If there are multiple or none, an [`InvalidInputError`](@ref) is thrown.
+"""
+function momentum(
+    psp::AbstractPhaseSpacePoint, dir::ParticleDirection, species::AbstractParticleType
+)
+    _assert_one_particle(particles(psp, dir), dir, species, Val(0))
+
+    return momentum(psp, dir, species, Val(1))
+end
+
 """
     momentum(psp::AbstractPhaseSpacePoint, dir::ParticleDirection, species::AbstractParticleType, n::Int)